Besides things like what Qubes offers, is there any difference if I'm using Manjaro, MX Linux, Mint, Debian, Ubuntu, etc, when it comes to security and privacy?
Absolutely. No matter what Linux one uses, if the user ignores good security practices, then the user will be on the losing side. Doesn't matter Manjaro, Arch, Ubuntu, Qubes, Kali, or any other part of the alphabet soup of distros.
If you use any Debian distro security will be the same. Qubes is good, but just think, if you use only Linux or Linux/Windows too. For Windows Qubes is quite complicated. Probably Qubes or Debian is the best.