My biggest problem has been that I find nearly all of the MPV-based players I’ve used (mpv-player itself, or stuff like IINA or Celluloid) either have a massive lack of configuration options, overly minimal UI, or both. On Windows, Mac, or Linux

Have you checked the manual? The amount of options is staggering, bordering on esoteric. The only catch is most of them need to be setup either through CLI or .ini files.

But it’s weird that so many players don’t offer you the ability to enable basic plugins. Or change playback speed. Or configure keybinds. Or continue playing the next file in a directory automatically. Or even offer the ability to display the most basic info about the file being played, like codecs. Etc etc.

I can't say anything for the frontends, but you can use plugins for basically anything in mpv itself, including the issues you've listed. Again, you need to setup the whole thing and read the manual, but it's there. And yes, I see the irony in all the effort needed to do basic stuff like this when something like VLC just works™, but to me that's the beauty of mpv, it's basic and minimalistic yet you can make it work however you like given time and patience.

Btw you can check the file info by pressing i on the keyboard while playing something.