..situation again. Have used 21 as runtime for some months but trying to bump build target to 21 too causes new problems.

I feel your pain as a fellow "large application" maintainer.. Getting from 8 to 9+ took ages here (mostly javax/jakarta hell). Once it worked we still built for 8 but used 17 during runtime. When we finally tried to build for 17 a bunch of new blockers showed up. Looks like we're in the same..

Upgrading from which version? What makes it painful in this case?