

The straw that broke the camel’s back for me is the CEO’s icky tweet about how great Republicans are for your privacy and how they stand up for the little guys (what), which they doubled down on using the official Reddit Proton account. There’s already been a ton of discussion about this on the internet if you care to look for more angles on it.
But before that I’d already grown quite leery of them for their trend of endlessly starting new services before the old ones are polished, along with trying to push everyone into their walled garden and endlessly using naggy popups in the UI about it. Worst of all, they have a clear trend of not giving a damn about Linux support, sometimes giving up on certain features for their Linux clients or releasing the clients way after the Windows/Mac versions. For a “privacy company”, not putting Linux as a first-class citizen is really just unacceptable, and they’ve been around for long enough that it’s clearly a trend and not a fluke. To me, Proton just feels like a wannabe version of Apple. Its continued actions give me the feeling that it exists to serve itself, not its users.
Mailbox.org is a good pick to consider IMO. You can read some comparisons on PrivacyGuides, which I also recommend as a starting point for these sorts of topics. The mailbox.org web UI is not great, but it allows IMAP/SMTP access, so I use Thunderbird on both desktop and Android in order to interact with my inbox. My inbox is auto-encrypted with PGP using their Mailbox Guard thing, so my emails are all encrypted garbage on the web UI anyway. Mailbox.org only allows paid-for accounts, but considering the annoying stuff that Proton and Tuta do to their free accounts I’d rather just be honest about the service I’m getting. It allows auto-forwarding directly in the web UI, but given that you can hook up to it with IMAP anyway, it’s not like you couldn’t just do it yourself.
(Also, as another comment said I also recommend DuckDuckGo’s Email Protection for email aliasing if you need it.)