The worst thing about cross-compiling Qt is that I always run into different issues with new releases, and it's not even always consistent between builds. I ended up renting a 64 core VPS to build Qt so that I could iterate faster though build failures, but it's still at least half a day's work to get it to build properly.
Also: I'm assuming the 64 core machine is ARM? Which provider do you use for that?
It might be helpful though for 64 bit ARM, but my target architecture is 32 bit and because I need webengine I suspect that it will not compile on a 32 bit platform. The problem is that the linker requires more memory than can be addressed in a 32 bit address space for linking chrome/webengine.
I just used google cloud.