Is it easy/possible to install the Bazzite tweaks on top of Aurora?

Yes, most of these should be available via ujust

Also, my desktop has an Nvidia card… should I expect problems?

Bazzite (and all UBlue-based distros) specifically ask you what GPU you use before you download the ISO, and they have special images for Nvidia users.

apparently no game mode, which I don't use on desktop anyway. it's basically just big picture with some differences in the nerd shit nobody normal cares about, and I think big picture might have been updated to look the same too.

No, absolutely nothing wrong with that. Just want to let everyone know that it's not some random distro, but it actually uses a stable, reliable and well tested base distribution.

Note the difference between Fedora and Fedora Silverblue. Fedora is a regular Linux distro, like Ubuntu. Silverblue is an immutable distro which uses rpm-ostree and provides atomic updates. Universal Blue is based on Fedora Silverblue whereas Nobara is based on normal Fedora. Bazzite is a more gaming-focused variant of Universal Blue. I prefer Silverblue and Silverblue-based distros, because they are more stable and reliable. Bazzite perfects this experience and adds some more stuff on top, to making everything easier and more pleasant.