No, please do not do this. Two factor authentication should be just that: two separate factors of authenticating yourself. Having them combined in one is the same as one factor.

Proton VPN does not officially support Raspberry Pi, but you can try to set up their CLI version. https://raspberrytips.com/protonvpn-on-raspberry-pi/ I tried doing this a while back but it hung on the login step. I reached out to Proton and their only response was that Raspberry Pi is not supported. So, best of luck and I hope you can get it to work!