I spent so much time with this (try to compile a actual version of QT). I think it is not impossible, but for every step when you try to compile something else is outdated. Then you try to compile that ... next problem... and so on.
For me it is also hard to accept, that this smooth running OS is now outdated and even by open source projects no longer supported.
I love MacOS or better I'm really deep used to it. If you just got the old hardware but flexible with your OS you can give Ubuntu or better an lightweight Linux a try. Most of them running totally great on older Mac's and it's crazy how fast they are.