Puzzles are good but I recommend trying to get advantage in 3 phases of chess. Either in Opening,Middlegame or Endgame. Good luck with solving puzzles( or with without the 3 phases)
New Training idea - game positions

Is that a selection of your own games?
Initially I am going to use positions from games Master games from the openings I use - perhaps latterly I will add my own games: if my time only allows me to do 5 such examples a week, Master games would be best (I think), if I find I can do 10+ maybe choosing 2 or 3 of the positions in my games might be good practice.

I had similar thoughts, but I think this is only helpful if we are playing a critical position.
Do you think it needs to be a critical position? My thought process is that I seem to be OK at puzzles, where I KNOW the position I am presented with is a win (or in rare cases a defence of an imminent mate); I was thinking that the randomisation (sometimes key, sometimes more requiring just a developing or position move) would give me better practice at more logical thinking, and indeed an overall better thought process.
I wonder if this might be something I do, and then feedback to tell people whether is has been any good! I am going to continue to do puzzles and training, I just wonder if applying the same processes not knowing if an instant win is there might be useful (especially if I choose positions in middle games similar to those I find myself in).

Since a month I do my own games and take the positions were the engine changes plus or minus 3. In other words, the obvious mistakes. I have generally 1 or 2 of those mistakes per game on my side alone.
Time consuming but fun and very educational.
I think is not a very good idea. Engines doesn't tell you which is the best plan, just the best move. And it can confuse you a lot. This only works with puzzles.
The easiest way is to take chess lessons with a professional coach. There are good options here on chess.com.

As others pointed out, I don't think this is a good idea. You would have to analyze random and new positions without context not knowing if there is a clear best answer. Many positions don't have a clear best move, and analyzing a position afterwards can be very time consuming. Puzzles work better for that.
If you want to study interesting and relevant positions, my suggestion would be to mark (highlight) up to three positions each game during the game. I think your judgement of the relevance of a position is highest while playing and you know exactly what positions you find difficult to play. Then, after the game, you can do a deep dive into the position and discover what would be the best move.

I have an idea for a new aspect of my chess study: briefly, I am going to randomly choose a game from a selection, then randomly choose a move - and then from that position, write out the key points, and a few variations to 3-5 moves - and then check with an engine to see how I did, then check back to the original game.
Does this sound any good*? I enjoy puzzles, but with puzzles you know there is a win there, looking at positions which could be anything may be more instructive.
(The way it works at the moment, is a bit techy - it uses a command line tool - but I can streamline for other people to use if this approach seems valid, and anyone else is interested.)
*The reason for this is I bought a book, which ostensibly talks about studying 5 or so important positions every week; which to me sounded great, but the fist example has one side a piece up, and then the 'answer' is about 20 or so moves, which ends with 'could lead to winning the pawn on f6' (already a piece UP!!!) - so I think the book must be above my level, bad, or both; hence I thought coming up with my own positions as above might be useful.
Yes, I've seen a number of coaches recommend this. It totally doesn't need to be a critical position.
The only advice I have to give is to pick a move that you know (say, move 20?) that is decently past the opening. You want stuff that's solidly in the middlegame.
The idea is highly recommended -- go for it!
I have an idea for a new aspect of my chess study: briefly, I am going to randomly choose a game from a selection, then randomly choose a move - and then from that position, write out the key points, and a few variations to 3-5 moves - and then check with an engine to see how I did, then check back to the original game.
Does this sound any good*? I enjoy puzzles, but with puzzles you know there is a win there, looking at positions which could be anything may be more instructive.
(The way it works at the moment, is a bit techy - it uses a command line tool - but I can streamline for other people to use if this approach seems valid, and anyone else is interested.)
*The reason for this is I bought a book, which ostensibly talks about studying 5 or so important positions every week; which to me sounded great, but the fist example has one side a piece up, and then the 'answer' is about 20 or so moves, which ends with 'could lead to winning the pawn on f6' (already a piece UP!!!) - so I think the book must be above my level, bad, or both; hence I thought coming up with my own positions as above might be useful.