There's a bunch of open source successors (e.g. they started as Total Annihilation clones) made on the springrts engine that you may be interested in. I believe both Zero-K and Beyond All Reason are still being actively maintained.
I just bought it on steam a few months ago. $5. On my 10 year old system I had I think there was thousands of individual units running at the same time. I downloaded the mid Escalation with amazing unit, and was playing against 4 AI. At one point there were thousands of units all running at the same time and the battles were epic. I would throw 500 Fidos (my favorite original unit) to the AI and they would slowly get mowed down, but then I would
Keep sending them in waves. It’s still so much fun!