How do you want to handle testnet and signet(s)? Perhaps ._bitcoin-testnet-payment and ._bitcoin-signet-payment?
I have this itchy feeling of wanting to avoid the word “bitcoin” in both the subdomain and the record, to make both filtering and creating a map of all domains “involving” bitcoin require at least a few more seconds of effort.
Why use BIP21 style URI’s inside the text record? And why have a single big record for all payment types? For example instead of matt.user._bitcoin-payment.mattcorallo.com. 3600 IN TXT "bitcoin:?b12=lno1q... you could have matt.user.b12._bitcoin-payment.mattcorallo.com. 3600 IN TXT "lno1q....
I guess one argument could be that using a single big record makes it hide what the payer is looking for. But you can just query all of them. Or is a single query easier to implement?
One record per address type potentially avoids having to deal with the 255 character limit for TXT records.