Cheers for the pointers, I’ll give them a try.
This is a Qt6 build I’m trying, hence it being different to the GitHub action. Though I will note that my PR with Qt6 fixes has passed all the automated building tests including the Qt6 ones.
I’ll also have a look at my fixes and see if any of them were actually the opposite…
I’m assuming because you wanted to make sure this doesn’t happen again? If so, don’t worry, as like I said, this is Qt6. With the patched rapidjson you mentioned, the normal Qt5 build works fine 
As it goes though, I’m on openSUSE Tumbleweed.
But if it was that you wanted another reference system to test automated builds against, then maybe Tumbleweed would be a sensible one to add, it being cutting-edge with the latest everything, but also pretty stable due to the testing framework.
Tumbleweed plus the Ubuntu actions (Ubuntu-20.04 and Ubuntu-latest) that are currently in place would provide good checks on how the building proceeds on distros with different degrees of recency. It would help to catch things like the rapidjson issue! Plus it would then match one of the systems that Avogadro is being developed and tested on, I guess.