Signal accounts do not require a SIM. There is no requirement that the phone you use for running the app Signal has the phone number you use for Signal login.
My Signal number is a Google Voice number that has nothing to do with any mobile phone. The Google account has advanced protection turned on so you can’t port it or get the SMSes without a hardware login token.
It's still associated with a credit card and your google account requires another phone number to create.
In my country I cannot buy a SIM card / phone number without giving my full identification.
Can you buy a phone number from a different country? (genuinely curious, I live somewhere I can buy a sim card with cash, and saw some in the impulse-purchase section of a store earlier today)
So buy a number from a different country.
But has something to do with a bank card you used to pay for it?
That's cool that there are phonenumbers without SIMs, my concern was more about SIM swap takeover. (Signal only guards this with a 4 digit PIN iirc)
The PIN can be longer than four digits. Signal also guards against this with safety numbers; if someone takes over an account, every contact will see that the safety number has changed and should consider that the account may be compromised until verifying out of band.
Google Voice doesn't look like a safe option, your number can be taken away if you forget to pay or you can be banned for arbitrary reason without a way to appeal.