The 5600XT is also going to need Navi firmware, currently not available in any version of Debian. You'll have to get it manually, which a lot of people have trouble doing , it seems, or install the "linux-firmware" package I've ported from Ubuntu for MX Linux. There's a current thread here about a 5700XT on testing which is having trouble without "linux-firmware", though others have successfully used it here in other threads.
You may want to try the Live session of MX 19.2 AHS edition, which has all those backports already, just to confirm that it works. Then you can redo all our backports if you want.
Most of the build time for LLVM is for the self testing after the compile step, which can be disabled by an override in debian/rules if you know how. We don't disable those for the MX backports, though.
Anyway, creating an sbuild environment is pretty easy. Install sbuild from buster-backports along with ubuntu-dev-tools from main. Then, if you're on amd64 and need the 32-bit packages also:
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mk-sbuild buster --arch=i386
Then log out and back in to make sure you're in the sbuild group.
Check out sbuild's man page and the Ubuntu and Debian wiki pages for sbuild also--though some tricks still seem to be poorly documented, such as adding multiple outside repos to it, like debian-backports at the same time as the MX repos, but it can be done.