I never invoke mpv from my command line shell, I invoke it through my file managers (Dolphin or dired). I configure mpv using my text editor to edit the configuration file at ~/.config/mpv/mpv.conf. If I want to make on-the-fly changes I use my mouse and keyboard to invoke key bindings. If I don't have a keybinding set for whatever I want to change and don't want to edit my config for it, then I can use the built-in REPL to change it (but I very rarely have a reason to do this.)

> correct interface for a video player.

That 'correctness' is subjective. It isn't correct for you, but it is for me.