Let me start by saying that I don't think Proton is a bad company in any way, and I have no evidence of any kind of malfeasance, but I still must ask...
Do you have an exit strategy if Proton goes the way of Microsoft or Google? Those two only had apathy and inertia keeping people in their ecosystem, but Proton kind of has you locked in, because you can only use their proprietary apps to access encrypted emails. Yes, I know you can use IMAP, but my understanding is that any email sent via IMAP isn't encrypted, and any encrypted mail can't be downloaded via IMAP. Either way, just because they have IMAP turned on today, doesn't mean they won't turn it off tomorrow in the name of security.