I switched from Posteo to Protonmail 2-3 ago, and I have been quite happy since.
The reason I switched was mainly because Protonmail has many features that I missed from Posteo. And, at least in my experience, Posteo's spam filtering is really, really poor, and it became a real nuisance having spam daily delivered to my inbox. (Reporting these to Posteo's team did not seem to improve their spam filter). I do still pay for an account there, mainly to keep receiving email to the addresses I have registered there, but I have set up email forwarding to my Protonmail account. I logged in to Posteo today and immediately spotted a scam mail in the inbox. I then checked Protonmail, and it had correctly flagged the same email as spam.
Protonmail also supports custom domain names, which Posteo does not; although I respect their reasoning behind this.
When that is said, I do encourage exploring Posteo as an alternative to the other providers mentioned. If you find their web interface to be barebones, you can set it up with Thunderbird, K9, etc. etc. without issues. And their aliasing service is good. Protonmail can do basically unlimited aliasing as well, but you have to pay for their Unlimited plan, which is expensive if you only want their email service.
On the question of whether Protonmail is blocking Mullvad: Until a couple of weeks ago I used Mullvad with Protonmail for about 2-3 years. I have also logged in through Tor several times. I was never blocked nor did I have login issues, but then I have not been breaking their terms of service.
I have seen accounts on Reddit about Posteo also suspending accounts, and their support being unwilling to give a reply. I don't know how accurate these stories are. I have nothing but good experiences with Posteo customer support, but then I have never had my account there suspended.