Better not pay for hosting or anything related to that server or IP address with a credit card, because that links to your identity.

That is the only thing that was provided here. Proton even provides anonymous ways to pay, they just didn't use one of those methods, they used their credit card. Even if Proton didn't store any personal info directly for that payment, they would have a transaction confirmation for the payment, and the credit card payment processor definitely will have your info by necessity to process the payment.

Well to be fair, FBI would tap whatever VPS you use to host your email server and get your info from them. Email is not secure.