The kicksecure documentation is well written. Go here for distro-morphing of Debian 12, step by step things to do are pretty explicit and you can even copy the command lines if you think you'll make a mistake...
Does Kicksecure implement proprietary software into Debian? Personally me, I run OpenRC and the linux-libre kernel on my Debian 12 system on a Libreboot laptop with fully free software. Haven't used Kicksecure in awhile.
I really like Linux libre kernel but it has some downsides because it doesn't have patchs to fix some critical vulnerabilities. Libreboot is just a distant dream unfortunately few devices are compatible with it.
According to their Wiki, other Distros (including DE) are not supported/documented and if you do it's at your own risk.
Personally, I wouldn't dare trying that.
Kicksecure is a good choice for an install on bare metal. For a level-plus secure and private system you can even install a Whonix VM on Kicksecure, so you can use Kicksecure daily use your Whonix VM to get on internet. It's a great combo i thinkk, as they use the same base, the same DE (xfce) and have a lot of defaults programs in common.