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.

https://support.google.com/docs/answer/3093339?hl=en

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.