Well, yes and no.

I take it back that the open source drivers are just as good as NVidia's. The latter do give considerably better performance for games (I tested again Team Fortress 2, and Two Worlds running in Wine). Other than the performance (gaming), the open source drivers work nicely though, even having fixed the brightness issue I had.

If I switch to NVidia's drivers in the Driver Manager, the same problem happens and I start getting package errors etc., even though I've run e.g.
sudo apt purge nvidia*
sudo apt autoclean
sudo apt autoremove
sudo apt update
sudo apt upgrade
sudo apt install -f (this never seems to work...)
probably something else too...

Something is rotten in the state of NVidia...

I had horrible problems with Nvidia before, to the point that it was almost impossible to clean out all the junk the blob left in the system and it just refused to start X for me.

You might try manual Nvidia driver uninstaller, but results are not guaranteed. All of this is the result of Nvidia totally disregarding proper upstream driver development.

I really appreciated switching to AMD, and leaving all these blobs horrors behind.

The proprietary nVidia drivers is a must if you have recent GPU (post-Kepler, AFAIR) and want to play something more demanding than "The Battle for Wesnoth". :) This is so (partially) because on the newer cards nouveau can not reclock GPU. Needless to say, all fresh Vulkan and OpenGL versions are available only on proprietary drivers.

Using 390 drivers with fresh cards ("GeForce RTX 2080 SUPER"???) is pointless as old drivers do not recognize "device ID" of your card and just assume the worst (hence 1024x768 as from their perspective this resolution should be supported on anything) as the alternative is no graphics at all.

Use the most fresh 440.44 version. If it is not available from the get-go, add additional unofficial repositories, download rpm or deb files manually from some site like "pkgs.org", do any other magic necessary… Install the drivers manually from the nVidia site directly, if you must (don't listen to any "Gurus" out there telling you it is not right -- your PC, your rules)! But make the card work. Otherwise, it will be wasteful usage of (the most expensive) part of the system; you'd better get some Intel or AMD APU and use integrated solution. At least, the electricity bills would be smaller. :)