Note: Don't upvote all the answers at once because your votes may be reversed!
Part 1. Strategically, what's next?
Besides books (r/chess has a list), I'm looking for online courses or video lectures (preferably 'online, free and complete' as Hauke Reddmann says, but even if not that's fine. I mean, chessmaster wasn't/isn't free) that
have endgames that aren't covered in waitzkin's endgame series in chessmaster eg 2 rooks vs queen or knight vs knight.
have more examples of what the endgame course already has eg knight vs bishop, bishop and knight vs rook, rook vs rook, bishop vs same/opposite coloured bishop, etc. (given the drawishness of opposite coloured bishops, maybe we can skip this.)
Note: This has been asked on reddit but not for endgames specifically.
What I've found so far:
There's the Saint Louis Chess Club on youtube. Like I search 'endgame' there and then there a lot of videos, but it seems I'll get videos that don't necessarily follow some progression like in a course. In particular, they don't have a playlist for endgame.
1.1. There are other channels that have endgame playlists, but they don't necessarily seem to follow some progression too/either(/grammar?) eg Chess Club and Scholastic Center of Atlanta, Remote Chess Club
1.2. Edit: Idea: Maybe courses that don't necessarily follow some progression can be like a good supplement to other stuff that actually does follow a progression. More here: https://chess.stackexchange.com/a/36947
P.L. Chess Endgame Course: There seems to be a lot of either endgame puzzles, w/c I could just do on lichess (but of course wouldn't necessarily gain understanding from the puzzles w/c is of course why I'm asking this question), or 'theoretical endings'. But some of the videos on this list I guess do kind of cover what I'm looking for.
Part 2. Wait any more tactical or puzzle stuff?