If you're not comfortable mucking with the kernel: Install a dedicated media distro and add whichever DE you prefer. Otherwise, sticking with Debian will likely be more convenient.

If I want to install and use a different desktop environment than came with my distro, what (if any) problems can I expect to encounter?

I'm not knowledgeable about kernel tweaks, but switching DEs is very easy on Debian and should not cause any problems. Just install the desktop metapackage (e.g., "task-kde-desktop") and you'll be able to choose it from the login screen.

What optimizations are you wanting? Chances are you don't need a specialty distro. Those distros are often pretty obscure and maintained by only a few people.