IMHO, you need to do fresh install of PureOS Byzantium Live image under CSM (start there with what you are up to, by installing PureOS) from 20-Nov-2020:
$ seahorse --version
seahorse 3.38.0.1
GNUPG: /usr/bin/gpg (2.2.20)
Yes there is, but you are not to be allowed (for your own good) to have two main repositories active under one healthy OS (either Debian one or PureOS one).
And take a look, having this advantage, at your current Debian location of firmware files as you’ll need to rebuild the same ones under PureOS, actually search for and write down which non-free packages you’ll need to install afterwards (as same laptop will be used).
P.S. Otherwise I do not see reason why to be afraid of installing Debian firmware-bullseye-DI-alpha3-amd64 .iso image.