I am not a lawyer this is not legal advice.
Search for copyright assignment agreements, there are a few good template documents. I’d request a signed document instead of simply stating it in the PR. In all cases I would recommend verifying the document and process by a lawyer before you start using it.
Also, I would consider not requiring copyright assignment if at all possible for the project in question.
A lot of fun so far ! Thanks for making this
Open source Not […] allow […] commercial applications
For some definitions of open source, this is contradictory (FOSS). Some non FOSS licenses do limit commercial use while making the source available, this could be a possibility for you.
But, I’d instead recommend that you check if a copyleft license such as the GPL provide enough protections for your use case.
Maybe look into PipeWire, Reaper in jack mode works fine connected to PipeWire for me, without any of the problems you mentioned.
It is quite possible that your desktop audio already uses PipeWire depending on your distribution, you would need to look at latency settings if you want live playback.