But, I don’t care at all about things like “hosts files”,
“cosmetic filtering”, “popup blocker”, “zapper/picker element” etc
etc etc. I do care about RAM, system performance etc. In this
context, is there any reason to prefer UBlock rather than UMatrix?
No, you are definitely better off with uMatrix in such case.
I am back using uMatrix myself, hence why I am currently dedicating
more development efforts to it.