Please add a plan like Duo/Family Unlimited but only for Proton Mail (not Unlimited).

My use case: I want to move to Proton and share a custom domain with my family. The only way to do this is with the Proton Duo/Family "Unlimited" but these plans offer a lot more and in consequence are more expensive. Because of this I would have to pay double the price of Mail Plus for features I don’t want to use.

Current:
Mail Plus for two is $10 ($5 each)
Proton Duo Unlimited is $15 ($7.5 each)

What I’m asking:
Proton Duo Mail Plus: $8-10