I would say that the more important reason is to not have all your eggs in one basket. If Proton is your main email then it getting compromised would also mean your password manager being exposed. Just to not over rely on a single point of failure I would require at least different passwords. An independent TOTP for Proton Pass would be ideal.

This is exactly my situation.

This is a pretty good point and partly why I will not move away from keepass. But it doesn't invalidate it as something I'd strongly recommend to my friends that would otherwise just use the same password for everything.

I use a password manager so that I won't have to remember my password to emails and such. Hence, using the same password for the manager and for the proton account defies the point.
Additionally, it is bad practice to have the same password for two online services

This would be invaluable, I'm in a very similar situation. Lack of ability to sign into Proton Pass using a standalone set of credentials is blocking me from migrating from Bitwarden full time.

Yep, would be great unlock our vaults with a password locally, so that even linked to our Proton account, we can use a proper password on ProtonPass.

I so agree with "ohoh" comment below. Too much concentration is not a good security way.

Like on Bitwarden, let the user choosing for password unlock (rather than PIN) on web and smartphone versions.

Like on Bitwarden, allowing to create a master password, uncorrelated to the our Proton account credentials.

Much like other services offer, using a master password to open the vault is a must, it's way more secure than using a pin

EDIT: Users are logged out after 3 failed attempts to protect against brute force attacks

I don't use ProtonMail as my default mail service, therefore my Proton account is protected by a generated password that I can't and don't want to remember (using Bitwarden).
Having a way to use a completely different password than the one of my proton account to unlock Proton Pass will be a dealbreaker for me.