i almost did ths today, but reconsidered when i saw in apt list --upgradable various -puros packages were going to be replaced with (slightly) newer debian ones
not sure how to go about restoring this, though, unfortunately—but there must be some command to “downgrade” the installed packages to what is in the repository, after removing the buster lines in apt sources
(another option might be to wait for the rolling release Librem is working on, would suppose that’d track buster more closely)