Compiling qt on the pi is an overnight sort of thing. I think it took 12+ hours when I did it on a pi4. Thankfully, once it's compiled, you don't have to compile again until you next version bump.
I think people working on cross compilation are wasting their time. They are trying to fix it "properly", separating build time dependency and run time dependency, because host system can't run cross compiled target binary. The obvious solution (what Scratchbox2 does) is for host system to run cross compiled target binary, instead of trying to "fix" the build system.