Zero-K - completely free, very advanced and satisfying RTS made by gamers, for gamers.
Zero-K is a FREE multiplatform, open-source RTS game which aims to be dynamic and action-packed.
I tired this out a while ago but never quite got into it. Maybe it's improved since then, I'll give it another shot!
The website emphasizes the multiplayer features, but I was happy to see it has a single player campaign mode too. Tbh that’s usually all I play in RTS because I’m not that great at multiplayer.