Between 1800 and 2300, both positional understanding and technique kick in (hard !).
...and still calculation, but here it may vary from individual to individual.
I analyzed one of the games posted, it was quite fun! If anything these are great for working thinking process, calculation, and evaluation:
Do you think the old addage 'A bad plan is better than no plan at all' is OK for sub-1800 players?
Hesitantly, I'd say yes. A plan involves a coordinated effort for activity. This should at least give you practical chances against an U1800 opponent (even if your position is objectively worse). The alternative of passive / damaging moves is of course worse... just be sure your plan isn't disastrous as in hicetnunc's game 3 where black may have decided to attack the kingside with 25...g5.
Just some very general advice. When you pick a side of the board to play on, look for some of these: more space, more pieces participating, and targets. Preferably you'll have all 3.
Note undeveloped or poorly placed pieces aren't participating. By participating I guess I mean influencing that half, or more specifically, that quadrant of the board. Examples of targets include weak pawns, holes, undefended pieces, and loose enemy king.
Using this criteria, Black's "plan" with g5 involved nothing good for him, and in almost everything I listed good for his opponent.
Also with this criteria in the 2nd game notice all of black's moves after the opening (except a6, which was a waste of time) focused on the kingside and center. White, to his credit, also focused on the kingside, and in spite of some clumsy defensive moves he still had chances far into the game.