Choosing GM to study


"I really love to play slow-paced, sound, solid positional game..."
One GM whose play might fit what you are looking for is Ludek Pachman. His play was very principled, so figuring out why he made a move isn't that tough. Beside's Capablanca, Karpov's early games are also pretty straight forward.
For a tougher test, Petrosian, Fischer, and Carlsen are often very strategic but can have deeper ideas.
For an ultimate test, might want to check out some top computer versus computer games and try to figure out the move principles they employ or break and why it does or doesn't work.

If your main goal is to improve then it's probably bad idea to get fixated into certain style. Unavoidably all players naturally develop some stylistic preferences but it shouldn't be forced. One should try to be as universal as possible.
If, on the other hand, you mainly consider chess as an art and value certain style above winning then forget about what I said above
"... there are major advantages to studying older games rather than those of today.
The ideas expressed in a Rubinstein or Capablanca game are generally easier to understand. They are usually carried out to their logical end, often in a memorable way, ...
In today's chess, the defense is much better. That may sound good. But it means that the defender's counterplay will muddy the waters and dilute the instructional value of the game.
For this reason the games of Rubinstein, Capablanca, Morphy, Siegbert Tarrasch, Harry Pillsbury and Paul Keres are strongly recommended - as well as those of more recent players who have a somewhat classical style, like Fischer, Karpov, Viswanathan Anand and Michael Adams. ..." - GM Andrew Soltis (2010)

Earlier today I received an email containing this article :
https://thechessworld.com/articles/general-information/7-best-players-for-studying-strategy/
What timing.