I think open source software has the huge advantage of being auditable. I suggest you and your team audit the entire code to see if anything is harshly wrong in there or you rely on other people doing it with you.
We actually dont know how many backdoors are in proprietary software and we never will until all code is finally forced into the open as it should be.
I appreciate the elaborate response. The intricacies of licensing arent fluent in me and the reminder helps.
Copyleft is cool but for OPs question, I would suggest source available at least. My criterion is that I (or op for that matter) can look at the source code of this project, not everyone on every downstream project.
I‘d also distinguish between in and out licensing. If they want to make a product that is not foss, copyleft wont work so reviewing the code would be the smallest denominator imo although I would not use or recommend their software.