I've lost touch with RTS, but I'm pretty sure SpellForce should be amazing https://store.steampowered.com/app/311290/SpellForce_3_Reforced/
It does have a bit more RPG in it than Warcraft 3 though

AI WAR offers you the best long-term coop experience. too hardcore for me
Mindustry has an automated touch to it
8-bit armies is like reskinned command & conquer
Boid is free and amazing, pvp-only, but it's dead. it does have a campaign dlc though
Ultimate General and RUSE come to mind too