I would personally stay away from proton for anything, vpn or email, I’ve been following their news since started and a lot of sketchy things about them, you can read about some of it here (1), the CEO of protonvpn and Tesonet (data mining company) is the same person, they used to have a lot of vulnerabilities and bad patching system (2)(3). There are other stuff about it can’t find the reference for right now (hmmm?), but I can dig deep later if needed.
Mullvad has been better so far, I personally use / used it, last two years been really bad with their network, and recently they stopped port forwarding (4), but again, you have to trust them.
(1) https://encryp.ch/blog/disturbing-facts-about-protonmail/
(2) https://blog.talosintelligence.com/vulnerability-spotlight-m...
(3) https://www.scmagazine.com/news/network-security/protonvpn-a...
(4) https://mullvad.net/en/blog/2023/5/29/removing-the-support-f...