How to improve my early endgame / late middlegame? When I search for books on the middlegame versus the endgame, I usually find discussions about positions that have almost nothing to do with each other: the former being rich with strategies with upwards of five pawns on each side, and the latter being more precise and cut-and-dry, rarely with more than one or two pawns if there's a valuable piece on each side. Discussions of the latter sometimes include vague advice along the lines of "try to reduce to a winning cut-and-dry endgame" to extend the complexity a couple pieces/pawns more. Yet when I reach a point in the game like this:

6k1/5p1p/6p1/2Np1b2/1r1P4/7P/5PP1/3R2K1 w - - 5 28


I feel like I'm playing on nothing more than shallow tactical threats sometimes colored by old adages like Tarrasch's rule, the principle of two weaknesses, bishop and pawn colors, open files, etc. Can anyone recommend me sources of good theory/practice for this stage of the game?
It sounds like you are looking for strategic guidelines in non-theoretical endgames. Then a modern classic is Endgame Strategy by Mikhail Shereshevsky. Strong players tend to like this book, which should tell us something, but I'm not sure exactly what (causation? correlation?).

Another decent choice is Practical Endgame Play - beyond the basics by Glenn Flear. It focuses on so-called NQEs (Not Quite Endgames) where each side has a couple of pieces each, not counting kings and pawns. The chapter on Rook and Bishop against Rook and Knight contains 50(!) fragments from real games with this material balance, the one in your example. At least it should give you plenty of study material.