There is: Linux Mint Debian Edition. There are a few things you're missing in LMDE than in the standard Ubuntu-based version though, such as the driver manager and support for PPAs. The latter of which has declined in usefulness with the rise of Flatpaks, I haven't installed from a PPA in years now.

most users simply do not care. if it works, it works

Not too hard, especially if you plan on running the same software on your new distro. Basically, all of the settings are in your home directory (/home/[username]/), so you could just copy everything from your home directory and that's that.

Not only that, but you could also set up your home dir to be on another partition or drive. Basically, you don't have to copy anything if you set up your distro like this. You just point the new distro to your former home directory, this is home now, and it'll just use all of the settings from there. Sure, some settings and files are distro specific, but you can manually delete those if you want to free up a few MB of space.