Hi JoelStutz,
Thanks for sharing updates with us and I really appreciate your effort and your precious time doing those tests. First, please accept my apologize you meet such an issue and I understand it effort on your daily work.
I understand the inconvenience caused and apologize for it.
First please accept my apologize for late reply due to out of work
Based on my test result with different Excel version, I can confirm I can reproduce issue as like you described above with only Excel app on Mac OS. Even I tested Excel( insider fast) 16,58 version but meet same error message when I trying to "Save As" with "/" in the file name. See image beow for your reference:

Since, I also reproduce issue from my side as well, in this situation, to report this issue to Excel development related team, I suggest you provide the feedback via Microsoft Excel app, to development related team or via our Excel feedback portal so that they can get notice about it. It’s the best way to report to development related team and we are sure it will speed up this process. Your advice is very important to us. Refer to this article: How do I give feedback on Microsoft Office?
Thanks for your corporation and understanding. I appreciate your your effort and your precious time and stay safe!!
Best regards
Waqas Muhammad