Debian is Debian based and regular Mint is Ubuntu LTS based and use theirs respective repos (not a big difference for the average user). While currently the non Debian version is the main and recommended version, due some controversial changes in Ubuntu people want to move away from Ubuntu and the devs have considered making the Debian edition the main one.
SearXNG has maximum privacy and results, but it’s a bit too complicated for the average person. DuckDuckGo has worse results than google because of Bing base, Startpage is similar to DuckDuckGo, but it has as good results as google. Brave search has good results and is not reliant on other search engines.
The repo shows all the patches. It uses some patches from ungoogled chromium for privacy. It isn’t my recommendation here, I just mentioned it because Brave didn’t work for OP
Thorium is good for privacy and speed but not security, Vivaldi isn’t that private, ungoogled chromium removes everything google. Brave also has packages available for manual installation if you want to give it another try
No not really. Gnome for me is about 2.5 gigs of Ram, XFCE 700 megabytes and the CPU load also is way lower. XFCE can be heavy or light depending on how you configure it