which opening strategy?

Opening choice comes down to personal preference, and what type of middlegames youre comfortable with. So you need to ask yourself: "What type of middlegames am i comfortable playing?"

Even GM chess coaches debate this topic, but I can see the value in both approaches.
If an opening has a ton of theory, then even if it is objectively dubious (or worst, outright refuted!), it may be playable sub-2000 rating (based on the specific opening, some really objectively bad openings might not be playable even sub-1600 though), the opening in question might still be playable. To better understand which approach is best, I think it is first worth a little introspection on why you are avoiding the mainlines.
Playing an opening with lots of play even if objectively questionable at the levels of the chess elite (Grob Opening, King's Gambit, or the Englund gambit come to mind as openings that are perhaps playable sub-2000 level even if objectively questionable for super-GM level), is probably done to avoid the mainlines. It is a massive advantage you have if you play this opening 100% of the time and your opponent sees this opening perhaps 1 in 20 games they play or less. Thinking from this perspective, there is merit in playing less popular lines.
However, I'm personally the other way. I enter the deep mainlines and I tackle sharp theory head on! Does everyone and their grandma know these openings I play? Absolutely. However, I'm hoping I know them better. This has its own virtues as well, but by the end of the day: it is probably your decision based on what you are looking for and what you are comfortable with.
If you really get deep into heavy mainline theory, then it will probably serve you well through GM-level. If you choose something more questionable like the Grob (1. g4), then perhaps you may find yourself switching openings before you reach 2000 rating. Of course, you'll at least learn a lot of important elements by playing the unorthodox lines though - elements like keeping the initiative or piece activity.

Every type of opening has a lesson it can teach. Ultimately the biggest factor in chess improvement is that you enjoy playing chess and are passionate about learning more facets of the game. Playing an opening you love, that you can use as your artistic signature, is part of that.
One neat thing about engines is that while they may have killed off some openings they also opened the floodgates to playable but obscure variations. Anyone with an engine can map out novelties that are sound enough yet likely to trip up opponents.

Chess Openings Tier Lists by GM Hikaru Nakumura and IM Levy Rozman
Chess openings are rated in terms of their appropriateness vis-a-vis player skill level - i.e., beginner vs intermediate vs GM’s, etc. For each of the openings discussed be sure to pay attention to whether the evaluation is from White’s or Black’s perspective. They also frequently refer to some openings as "garbage" or "bad"; these qualifiers are used in the context of, for example, how much so-called "theory" (i.e., documented variations) the opening encompasses, or how much emphasis the opening places on positional versus tactical skills in order to play it well. For example, an opening they refer to as "garbage" (an unfortunate choice of terms) for a beginner may in fact be appropriate for higher rated players who typically know more of the theory for particular openings and have a more highly developed understanding of positional concepts.
The Chess Openings Tier List for Beginners…
https://www.youtube.com/watch?v=M9CwH47r6og
https://www.youtube.com/watch?v=fHsb7-LbC34
https://www.youtube.com/watch?v=z3FBRlzSMHc
The Chess Openings Tier List for Intermediate players...
https://www.youtube.com/watch?v=Qq_rEYTiLy4
https://www.youtube.com/watch?v=mCVdrmKHdiI
https://www.youtube.com/watch?v=Pblb8ZQ3OJ4
The Chess Openings Tier List for GM's...
https://www.youtube.com/watch?v=glMp0dNGPN4
https://www.youtube.com/watch?v=3vzDjlu-96s
https://www.youtube.com/watch?v=HShiBcGbfeA
The Chess GAMBITS Tier List for Beginners...
https://www.youtube.com/watch?v=S9N6Bo7BBPg
GAMBITS.....with moves...
https://www.angelfire.com/nf/chess/Gambits.html
https://en.wikipedia.org/wiki/List_of_chess_gambits
https://www.mark-weeks.com/aboutcom/aa04g10.htm
https://www.mark-weeks.com/aboutcom/aa04g31.htm
https://www.ianchessgambits.com/
Openings Guides - SimplifyChess.com...
https://simplifychess.com/homepage/openings.html
Chess Openings Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond
https://www.chess.com/blog/RussBell

Even GM chess coaches debate this topic, but I can see the value in both approaches.
If an opening has a ton of theory, then even if it is objectively dubious (or worst, outright refuted!), it may be playable sub-2000 rating (based on the specific opening, some really objectively bad openings might not be playable even sub-1600 though), the opening in question might still be playable. To better understand which approach is best, I think it is first worth a little introspection on why you are avoiding the mainlines.
Playing an opening with lots of play even if objectively questionable at the levels of the chess elite (Grob Opening, King's Gambit, or the Englund gambit come to mind as openings that are perhaps playable sub-2000 level even if objectively questionable for super-GM level), is probably done to avoid the mainlines. It is a massive advantage you have if you play this opening 100% of the time and your opponent sees this opening perhaps 1 in 20 games they play or less. Thinking from this perspective, there is merit in playing less popular lines.
However, I'm personally the other way. I enter the deep mainlines and I tackle sharp theory head on! Does everyone and their grandma know these openings I play? Absolutely. However, I'm hoping I know them better. This has its own virtues as well, but by the end of the day: it is probably your decision based on what you are looking for and what you are comfortable with.
If you really get deep into heavy mainline theory, then it will probably serve you well through GM-level. If you choose something more questionable like the Grob (1. g4), then perhaps you may find yourself switching openings before you reach 2000 rating. Of course, you'll at least learn a lot of important elements by playing the unorthodox lines though - elements like keeping the initiative or piece activity.
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I fully agree that main lines are main lines for a reason.
However, you speak of hoping you know it better. Here is a typical mistake most make:
Chess is not about who knows the opening "better" or "deeper".
If you can regurgitate 34 moves of Winawer theory and I could only regurgitate 26, that does not make you a better Winawer player. The question is, do you fully understand all 34 moves and WHY other moves are weaker? Can you put in words what is wrong with the dubious move?
If you and I whip out 26 moves of French Winawer Poisoned Pawn, and you proceed with your 27th move, and Black now goes out of book. Why was the "book move" more correct? If you cannot answer, your claim of knowing thru move 34 is useless! I deviate on you at move 27, and you fall apart!
If in the line 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Qb6 6.a3 Nh6 7.b4 cxd4 8.cxd4 Nf5 9.Bb2, you were to ask me what White should play after 9...Be7 and what he should play after 9...Bd7, I know the answer and I know WHY!
After 9...Be7, Black leaves the diagonal to his King open (a4-e8), and so d4 is not threatened if White develops his Bishop to d3, and he should! Now a triple-take on d4 drops the Queen to Bb5+.
9...Bd7 stops the check, and now 10.Bd3? Merely drops a pawn for nothing.
So 9...Bd7 is a much better move than 9...Be7, right?
Not so fast! With 9...Bd7, the h4-square is not covered. White has the strong move 10.g4! The center is blocked and so advances in front of the king like this are not a problem. The knight is dislodged and pressure on d4 is lightened.
This move 10.g4?! Is not good after 9...Be7 though as it can be answered by 10...Nh4, giving Black the desirable trade of Knights!
So let's say White plays 10.Be2 against 9...Bd7, I move I frequently face as Black at the Amateur level instead of 10.g4. What now? If all you did was remember moves 25 moves deep, you are out of luck!
For me, I love facing the inferior move. Now, I get to play 10...h5!, stopping g4 and keeping the pressure on d4, now forcing the Bishop of a passive role AND maintaining my f5 outpost. White should not allow both! As White, I would play 10.g4! there.
You may know the mainline 47 moves deep, but with White's deviation on move 10, it is understanding the ideas at White's 10th move that matters.
Now take the Exchange Grunfeld. 1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5 5.e4 Nxc3 6.bxc3 Bg7 7.Bc4 c5 8.Ne2 O-O etc. So on and so forth.
I have occasionally seen players play ...Nc6 without ...c5. Why? What is Black's idea?
My response? Beats Me! It does not matter if I can regurgitate 12 moves of the Seville Variation (That is the one with Bxf7+). If I cannot explain it in words, Black can at any point deviate and I would be at a loss. This is why I make no claim to understand the Grunfeld, is why I hate the Grunfeld, and why I never play it from either side. But I can regurgitate 12 or 13 moves! Does not matter. Someone else that plays the Grunfeld regularly only has it memorized to say, move 9, but he or she would still crush me if they actually "understand" the opening!
It is not about who knows the line deeper!