I’m not sure.
I don’t need extra storage. I don’t use Proton Drive (I store things in my server). The Proton Mail I keep it organized deleting the emails and doing external backups. I don’t use the Proton VPN out of the free tier (I prefer to use Mullvad or iVPN). The email is the only thing that retains me in the Proton solutions. My partner would be adopting the same approach, therefore I don’t see a reason to jump in a more expensive tier.
In summary, the Proton Duo, would be interesting for me if they would create a version of the Mail Plus.