I loved playing red alert and command and conquer online when I was younger. It seems like the genre has fallen off.

I recently discovered They Are Billions which I found to be a fun variation of traditional RTS. The goal is to manage resources to build up defenses and defend against large zombie attacks. Single-player only, but I've played it for over 100 hours!

https://store.steampowered.com/app/644930/They_Are_Billions/