Some B350m models did get Zen 3 compatibility. Not all, if I remember correctly, though I could be wrong. So whether it’s compatible I think is model dependent. Whether an old B350m has the VRMs for a chunkier CPU would also be a reasonable question.
I mentioned the R5 3600 because the prices on them are great. A 5800X3D does perform better, but I see completed eBay listings at $225+. They also needs a cooler. I see one 3600 that went for about $50 and several that went for $60, which isn’t too much more than a 16gb kit of DDR4.
I would definitely consider a Zen3 CPU for this upgrade, depending on budget.
Yeah, even the better boards struggled to break 3,000MT/s with Zen 1. They sure were fun to play with, though, one of the last times I felt like tinkering was actually getting me something.
I’d be curious how the more modern “default” 3600 kits do, I didn’t have a Ryzen system by the time they were popular and cheap.