As a note to anyone else also suffering this, here’s how to solve without reinstalling MIAB:
If you still have web hosting ability, you need to make the following available via https:
https://mta-sts.yourdomain.com/.well-known/mta-sts.txt
You can use certbot to get a certificate for that subdomain.
You need to point a DNS A record to the mta-sts subdomain.
You also need a DNS TXT record for _mta-sts with: “v=STSv1; id=123456789;” (id can be anything, the idea is that when the number changes, it signals a change in the mtasts policy)
Within 12 hours, gmail was delivering to my domain, now hosted on protonmail.
The contents of mta-sts.txt (ignore parens):
version: STSv1
mode: none (this is the critical part - you’re turning off mta-sts with this)
mx: mail.yourdomain.com (or wherever you’re pointing your mx record(s) to now. Can have multiples of this line)
max_age: 604800 (or however long you like, in seconds)