Proton apps are not in int PureOS store.
Experience installing and using Proton Mail, VPN, Pass for Linux please?
proton apps work fine, I grab the proton vpn arm64 nativeish(need to use mobile settings compositor to squeeze the GUI) app .deb from ProtonVPN and for the rest make a web app, that works for me; you have to turn on Website Data Storage in gnome web. In theory I would like to sync calendars like I had working on my N900 to the gnome calendar, but that doesn’t seem possible on free tier.
I always think about Valves Proton when someone speaks about Proton (especially on title), until they write “mail” and such in additional.
They were started by some people working at CERN(the nuclear and particle accelerator research in Europe consortium) in the post hushmail(in the US) refusal to hand over the the FBI and choosing instead to purge the server HDDs.
I know that company, it is just less present than the gaming software, so my first thought is about the more present one, that is all.
I fixed it for you. 
In the context of this forum, I think there is a reasonable expectation that the reference is to Proton Mail.
I can’t answer your question but it may help to say what parts of that you intend to use.
Do you only intend to store passwords?
Do you only intend to do TOTP (time-based 6-digit generated codes)?
Both?
For strictly passwords, the existing native Passwords and Keys application works fine on desktop/laptop. (In a more formal sense, the GUI is called seahorse but the underlying service is called gnome-keyring-daemon.)
On the Librem 5, that service is activated and usable from other applications although the built-in GUI may need scaling. For example, Geary (mail client) on the Librem 5 stores its passwords using the Gnome Keyring service and that works fine.
Regardless of any of that, you need to weigh up carefully the pros and cons of storing confidential information in the cloud. If you actually need to access that information from a range of devices no matter where you are, you could decide that it is worthwhile. (My own personal choice is to keep confidential information local and to avoid dependence on an external party.)
It should go without saying that no matter what solution you use, local or cloud, it is only as good as the password or other technology that protects the encrypted container. If you have a weak password then an encrypted container is worse than not having one at all because the compromise of that weak password then compromises all of the secrets in the container.