Can't help you with AV but otherwise your issues and confusions are all Ubuntu and Canonical and nothing on there is representative of other Linux dists.

Ubuntu is highly opinionated. Great for some/many people but not the best fit for everyone or even an obvious recommendation for newcomers (anymore). For your consideraion: Mint is basically a project that repackages Ubuntu to adress those issues to make it accessible for people not onboard with the Ubuntu idiosyncracies and more casual users who just want their desktop. Should be an easy migration for you.

Your Vivaldi problem comes from that you trusted gpg key for their stable. release repo, and fail verifying package from their archive. repo. Change repo to stable (that's prob what you want) or get the key for archive.

Your Ubuntu experience as told is not representative of desktop Linux experienced outside of Ubuntu. "But Linux sure could work better" is a misleading conclusion to share when that's all you know.