(Apologies in advance if some people have already hit on some of these points before, I didn’t have enough time to read through all the replies)
Good choice! Just be mindful of some of the potential security drawbacks.
I’m not sure why you would disable ResistFingerprinting. If you’re going to use LibreWolf, it’d probably be best to stick to the defaults as best as you can. If not, then you’re probably better off using Firefox or Brave with their respective recommended configurations.
Furthermore, Mullvad Browser just seems overall much better than LibreWolf. Even though you’re using ProtonVPN, you should still get pretty much all of the benefits with Mullvad Browser. The only thing you’d be missing out on is their proxy features with their browser extension, but with LibreWolf + ProtonVPN you’re already missing that regardless. (Well… that and the Mullvad Leta search engine which I’m not a fan of)
Another consideration is security. Firefox seems to fall behind Chromium in regards to security, especially on Linux and Android. Personally, I love Firefox too much to abandon it on desktop, but if that’s a concern for you, Brave might be a better choice for you. Just be sure to install the Brave RPM rather than the Flatpak, as per the recommendation of Privacy Guides.
That should be fine. Of course, it also depends on the instance(s) you use.
That’s also fine. Just be aware of what VPN services are actually useful for. There’s a lot of disinformation surrounding what benefits there are to using a VPN service.
It’s recommended to stick to using the DNS provided by your VPN service whenever you’re connected to their VPN servers.
If you’re looking for a new phone or mobile OS, I’d recommend sticking to Privacy Guide’s recommendations. Assuming you’re unable to afford a new Google Pixel to run GrapheneOS, DivestOS would be the next best option if they support your device. There’s many reasons to avoid LineageOS, and depending on your threat model, you might be better off sticking with the default distribution of Android your phone came with if you’re unable to switch to DivestOS.
Would you be able to elaborate on why you’re required to use Gmail?
(Sort of unrelated, but you should be able to use another email client if you want. It should at least allow you to store a local copy of your email. It probably doesn’t provide much of a privacy benefit, but it does mitigate your reliance on Google by a little bit.)
You can always use Cryptomator!