Frankly, unless you have a compelling reason to need Catalina, I'd regress back to Yosemite or Mojave, which should resolve the performance issues. I'd be on El Capitan still, but I wanted to learn how to do development with Xcode and Swift, and I needed a relatively current environment to do that.
I can't comment on the 16GB vs 8GB issue w.r.t. screen blackouts, only because I can't see why the one would have anything to do with the other, unless the added memory chips operate with different speeds. Whenever I add memory, it's always within the published specifications and always the same IDENTICAL chipsets. So, if the specs say max is 8GB, I won't try to add more.
When I first upgraded to Catalina, not knowing at the time about APFS, the system took ages to boot, and then ages more to open any apps. Mail was essentially unusable, and it wasn't until I went to the external drive route that I saw performance close to what I had seen on El Capitan.
Corey pointed out, in the thread I had going with him about my upgrade issues, that macOS does a huge number of tasks when booting up a new system; file indexing plays a role. Do a search on "Catalina Performance Issues" to read the full details of that experience (and to admire Corey's patience). But, IMHO, the whole idea for upgrading is to take advantage of new features that enhance your computing experience, not to sit around wondering when the computer will allow me to use it.
I have a Ubuntu Linux system running on an old Dell Optiplex 790, I think it is at 20.04 LTS. The last Linux I played with (at work) was 12.04 LTS. I was surprised, and a little disappointed, to see how little improvement there has been between the two releases. And although Linux folks make a big deal about asking the online community for help, in my experience, no one seems to have the same problems I do. I've played with SuSE and Red Hat, and I kinda favor SuSE, but it's been decades since I played with the lizard ...