cinnamon, lxde, kde, and mate are all desktop environments that mimic the basics of windows ui like the taskbar, start menu, and windows with minimize, maximize, and exit buttons at the top. and all can be installed/selected during install on pretty much every popular distro. there's prolly some others i've forgotten but those are all solid choices.
edit: whoops had mint listed but that's not a de that's a distro
don't know why you'd want to? you may trust your dns server but without dns over https the dns requests themselves are sent plaintext and are vulnerable to man-in-the-middle attack. with dns over https the dns requests are encrypted and that encryption would have to be broken for a MITM attacker to see your requests. more security is better and dns over https costs virtually nothing to use in terms of cpu resources.
edit: oh do you mean whole system mullvad VPN? if so, then yeah dns over https doesn't really help much but it's also still a case of why bother turning it off when there's no benefit to it.