I would like to change my Gmail for Proton Mail. The monthly option for Mail Plus seems reasonable to me. The cost for Unlimited seems a bit too high. I don't like paying 1 or 2 years in advance.
Proton Support said it is impossible to combine Proton Mail Plus monthly with the Pass Plus for 12 euros/year.
Their staff posted on their Reddit channels that it’s a limitation of their billing system, I believe in relation to having two different services at once. They said they’re working on updating the billing system to support this but I don’t recall an ETA.
I think you’d have to create a second account currently, but then you may lose access to the introductory pricing to Proton Pass if later you wanted to move back.