Then I could buy a pre-paid SIM card for a dollar anonymously using cash for the sole purpose of receiving this SMS verification. Or try various disposable email address for verification. Or I could email them and directly request an invite, since the only reason they require verification is to prevent spammers, and it's highly unlikely a spammer is going to email them directly for the purpose of opening a single account. Or I just pay the $5 to open a Plus account for a month, not a big deal.

Although if you are going to encrypt the PSBT before uploading it, you don't even need to use ProtonMail - just host it somewhere you can access via Tor and it won't get deleted. Open a GitHub account and stick it on there. You could probably open a topic on the Archival board here and stick it there. Message it to yourself using an encrypted communication app like Signal. There are multiple possibilities.