Tactics trainers:
I would imagine at some point the tactics are not the bottleneck and I should be working on something else instead (openings, endgames, or studying mid-game strategy?) but just wondering where that point is.
For lichess at least, they don't. I'm a 2100 tactics and nowhere near that in actual rating (around 1500).
For a typical player, the chess.com puzzles rating is at least 500 points higher than the blitz rating on this site.
I think tactics shouldn't be on focus only for very strong players, such as FMs or IMs, but you definitely don't need to choose one instead of another. It's fine to learn some tactics, then some strategy, then some endgames, isn't it?
Among players who have played 100+ games/100+ puzzles, the difference is -205: puzzle rating substantially higher, standard deviation 320.
I'm not sure that it's ideal for ratings to be identical across the domains, but they should correlate to some degree.