I'm using protonmail for 5 years now, everything works fine. Always.
But what's the point of registering to PM via tor?
They are probably blocking it.
Just register it regularly or if you want to register to PM from not your IP, use VPN, Proxy, whatever it works with.for anonimity...
how do you suggest to approach it instead?
sometimes they don't allow you to create an account through the Tor network.. try getting a new identity and try again until it works, or get a burner phone number to verify( purchased anonymously using something like Monero.)
https://juicysms.com/Never used this one before, but heard good things about itdo you know where to buy a burner with Monero for US number?