A FM needs training help too tl;dr: How to visualize and assess if you can't?
I'm pining, no, not for the fjords, but for returning to the board after Corona. I still read a good amount of chess books, but maybe some specific training couldn't hurt in the meantime.
Not surprisingly, as a FM I exactly know my weaknesses:

1 ageing (I'm 60 now)
2 no knowledge of opening theory
3 a certain shortcoming when dealing with pin tactics
4 horizon effect

When dealing with that...

1 I'm currently going for my diploma in computer science, so probably my brain's still flashing like it did when I was young (ha, Rolling Stones reference). Nothing specific I could do anyway except constant practice.
2 Remembering and playing main variants would take the fun out of chess for me (and it's even doubtful if it increases my score - what good is it losing 25% less by opening desasters when drawing 50% more?).
3 This could be easily trained specifically, e.g. with a Lichess pin themed training set.
4 This is what I am asking for (but feel free to comment on the other points). It seems I always calculate (when I calculate at all, my intuition is extremely good, i.e. I quickly see the candidate move if there is only one) exactly one move too short. (And that is when the figures begin to blur before my mental eye. I'm a very lousy &quot;blindfold&quot; player by the way.) For example, if the problem involves trampling the poor king to death with Q and R, the calculation tree forks to kingdom come and I am unable to follow it.

Thus: How to decide when one can cut a variant with a definite result? And don't just answer &quot;quiet&quot;!
A prime recent example (donated to me by a chess club colleague as a tactic problem for our club zine:
[fen &quot;&quot;]

1.e4 e5 2.Nf3 d6 3.d4 Bg4 4.dxe5 Bxf3 5.Qxf3 dxe5 6.Bc4 Nf6 7.Qb3 Qe7 8.f3 Nc6 9.O-O Qc5+ 10.Be3 Qe7 11.Nc3 Nd4 12.Qa4+ c6 13.Rad1 b5

Of course, I start calculating Nxb5 in a microsecond (no FM needed for that...). Hack, hack, hack, slice and dice, rook doubling, oh bother, Black can protect the mobbing victim on d7 sufficiently. Quiet. Cut. (Of course I'd play it anyway as the piece would be lost in any variant, but also of course I must already decide about that when playing Rad1.)
A check on Lichess with Stockfish shows Black is dead as a doornail: the position is quiet for one, two moves, but even if White could not play B-xa7-b6, Black would take ages to untie while White simply promotes one of the queenside pawns. Thus, again: how can I &quot;see&quot; that, even if I'm more or less unable to actually &quot;see&quot; the position before my wrong cut?
I'm not that old (yet), but I do know how it feels. I even showed a few of my games to GMs asking for feedback, and it turns out that very often I have seen the (good) move recommended by them, pondered about it, but could not calculate properly and played another move instead. Too bad.
The solution I came up with is largely based on Dvoretsky books (not the latest ones but those from maybe 80-s or 90-s): calculation is overrated. Calculating is for engines; for humans it is very inefficient in terms of energy consumption and error costs. Instead, you should avoid calculating as much as possible, and only do it when absolutely necessary. With your experience, you should trust your intuition and know which move is good and which not before starting to calculate lines. In your example, you already knew that Nxb5 was the move; there was no need to calculate it before you actually had the position after Qxb5+ Nd7. Dvoretsky actually has quite a few examples how to solve studies using this method.
This is not going to be a popular opinion due to the recent progress of engines chess; a lot of noobs will think that if computers play better than humans, they should try thinking just like engines do. I suppose if you're 15 this might indeed work for you.