I don't have a specific solution for you, but I also run a domain with some thousands of subdomains and is always a fight to not be banned from Google, Meta, internet operators etc. Sometimes is enough one bad actor under one of your subdomains to have a full ban on the whole domain.
What I suggest is for your and your clients to contact Meta through the Business Center support. Their support for paying clients is much better. I would also recommend you become a Meta Business Partner if Facebook/Instagram is important for your SaaS.