It depends on which option you think is the best, and if you care about centralization of your services. I would personally recommend Bitwarden in this situation to avoid centralization, but I wouldn’t consider it to be a bad move in the slightest to use Proton Pass since you’re already bought into the ecosystem. No matter which you go with, you are using a good password manager.
From what I can tell as both a Bitwarden user and Proton Unlimited subscriber as well, is that Proton Pass is very good, but offers no reason to switch to it from Bitwarden, other than wanting to centralize more stuff to your Proton account. I don’t like the idea of centralizing stuff to Proton, despite their products being FOSS and them respecting privacy and everything, just out of principle. To be clear, it’s a very well made password manager and looks awesome, it just doesn’t really offer anything that Bitwarden doesn’t, so therefore I’m not compelled to switch to it, because there is (in my eyes) only a downside of switching, being the centralization of more services controlled by a single company. I don’t think that’s too big of an issue though, and still am comfortable recommending Proton Pass to anyone who doesn’t use a password manager yet (or anyone who uses their browser as a password manager), just why would I switch to it from Bitwarden?
I’m already on 0.0.36, using F-Droid instead of the Play Store
An old Thinkpad is what most people recommend for Linux. Another good option is a Framework laptop, which is what I personally use and highly recommend! Another good option is any laptop from System76. Their systems come pre loaded with Linux, I think specifically PopOS, but don’t quote me on that part specifically. As for which distro to choose, there isn’t a “best” option for beginners per say. Those three distros you just mentioned would all serve you well as a beginner. If you want the best recommendation for all of your needs/concerns, I would recommend taking the quiz on distrochooser.de, but whichever beginner option you choose should serve you well. Hope this helps!