the opening and endgame databases

It is true that there are theoretically good moves, and bad moves in the opening, but this is true of the entire game of chess. The existance of the opening database, and memorizing lines really just makes it a bit easier on the chess engine, and the player -- processing each move to such a great extent isn't necessary if one knows what move is safe beforehand -- but that still doesn't necessarily mean that the opening is solved. If by "solved" you are referring to the win rate for a particular opening, then I would caution against that, as it can be rather misleading, depending on how exactly you are looking at it.

As for the endgame, I mean, there's a pretty small set of moves that are possible -- if you see a mate in 2, is that a solved game to take it?

along with pretty powerful chess engines

This point is moot -- are you using a chess engine when you play against a human player? I would certainly hope not.

There’s lots to memorize in the fairly rote early and late game, in the service of reaching an interesting and tactical middle game.

Why not skip the memorization, and, instead, put your faith in your ability to strategize in every phase of the game?