I recently discovered Beyond All Reason, a spiritual successor to Total Annihilation. There have been several of these, but I think BAR is by far the best. It's open source, but looks nice and is polished. It has amazing quality-of-life features I haven't seen in any other RTS. And it supports insanely huge battles with hundreds of players and tens of thousands of units.

150-player match: https://www.youtube.com/watch?v=gQJujiZ-oz0

https://www.beyondallreason.info/