Hi! While I am not very experience withe the issues you are presenting I hope this may give you some directions:
About biber: if the issue is only the package upgrade, then wait for it. You may want to open a pull request indicating this problem and the PR should just wait up untill it builds without any mayor problem. Or you could just update the template yourself and push it in the same PR.
I am not well versed with tlmgr… I think Void’s approach to other packages is to use them instead of the main Void repo (so no subpackages). However, this is not true for very wellknown utilities (see perl, python, etc) I believe LaTeX could get a pass with subpackages. But you need to ask the team.
About the PRs. While I haven’t done any, I have seen quite a lot of them. You should call them what they are: a musl fix for (insert package here). Then you can group the changes you have made into PRs, don’t make 10 PRs for different packages when they all belong to the same “group”. Just differenciate PRs when they are either too big or not so related.
Hope it helped you. Thank you for all the work!