Welcome!

I like a lot of the answers you’ve been getting so far, but I also wanted to add my 2¢.

I’ve been using Linux 2005. I’ve done some distrohopping in my time, but have I also settled into Linux Mint (Cinnamon). My system says it was installed in 2019, but I think I’ve been pretty much exclusively on Mint since ~2015/2016 after #! stopped being developed.

IMHO, you don’t have to do much customizing. I will never insult someone who wants more security but unless you’re running a server directly exposed to the web, I think the defaults and keeping your install up to date is enough. For the most part, privacy has to do more with the individual apps you run and once again comes down to personal preference.

Linux Mint is relatively lean, so optimization isn’t really necessary (I know some people will exclaim they can get a leaner setup by building up from a minimal install, and while they’re technically correct, I have no complaints with LM on a 12yo Thinkpad x201).

As for a “modern” UI, I’m not sure what exactly you’re looking for. I know on Reddit there were a bunch of distrohopping subs and other subs to share your desktop setup/customizations. Those were a good source of inspiration for me. I don’t know if there are some communities like that on lemmy. Personally, I just use the Mint-L-Dark theme, change my background, and use Conky. You can change the theme, use Desklets and Applets, or even use an alternative dock. However, I would recommend if you want to do much more than that, you should just try to another distro that’s designed with the DE, customizations you want. While you can run KDE/Gnome/etc on Linux Mint, the experience can be a bit rough. If, after trying out multiple distros, you find none quite fit your needs, you can choose the one that is closest and customize from there. Or, you can install a minimal/server build and work up from there (though that can also be painful, since you’re almost making your own distro).