You can lock the bootloader again after the flashing process is done(because it will add the signing key of the new OS), but unfortunately the NFC Payments in Google Pay still won't work because Google only allows it on 'certified' Android systems (aka only the preinstalled OS)
Yes, afaik LineageOS shouldn't be locked, but I was talking about GrapheneOS specifically because @wesker@lemmy.sdf.org was talking about that.