I tried building Qt6 on Linux, it was scarily fast. This I5 went into turbo mode, 3.6 GHz, OS/2 doesn't seem to enable the turbo and stays at 3.2 GHz, and temperatures shot up in this little computer, CPU at close to 90C (60C is about the highest it gets on OS/2) and worse, the NVMe at its max of 85C. Build failed in the webengine and I didn't want to do it again without more cooling.
As for cross-compiling, not only GCC and friends, the EMX toolchain need porting, a lot of other tools have OS/2 specific code. Think of the autotools. Plus there are test programs and such built during a complex build. Some use Rexx too. Possible but a bigger job then likely expected.