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.