Found the answers in the Bar FAQ: https://beyondallreason.info/faq/what-are-the-differences-between-bar-and-zerok

TLDR: Zero-K and BAR are based on the same FOSS engine. Zero-K is less micro with better unit AI. BAR delivers a more vanilla and micro intensive Total Annihilation experience.

Supreme Commander is proprietary closed source and has some different mechanics. Larger but worse maps.

More modern graphics and less unit types but more micro opportunity. Also zero k has buildable terrain in the form of ramps and walls. Bar has terrain deformation but can't actively do it with contractors.

Idk I haven't played Zero K but it's a good rts as well.

I was an extremely casual rts player for years playing mainly aoe3. Beyond all reason captured me instantly with the controls. Basically other rts games control difficulty with limited control scheme so you require high apm. Bar gives players as many quality of life controls as they want and allows for long action queues. I can queue up a unit to operate for 10mins while I focus on other parts of the game.

Bar also has basically no pop limit so you're only limited by the resources you control it also doesn't incentivize storing large amounts of resources. So wealthy is income focused. This means if you have more map control you have more wealth but because units drop a %of their cost in wrecks if you're behind and you win a good fight and can control the wreckage field you can catch up.

There is also so much more like the ability to transfer units, metal, energy to other players at 0 cost means working together is op. Strats involve a lot of "communism" and communication to decide where on the map needs the resources.

Example I might have won my side of the map and I'll send some metal over to another player who is losing so they can stay in the fight. Or my eco player will make units and sell them to players for their metal costs. Which means front players don't need to pay the metal and energy cost of building a t2 lab they just pay the unit cost.