My only complaint about Proton is it takes forever to start. Proton Mail shows a splash screen for seconds every single time. Over time it starts to feel unbearably slow. And recently, someone on their product team had the genius idea to put an app switcher as the new default screen, so now it's 1 click plus the splash screen just to get into your inbox. Takes too long to get to email basically, but on the plus side it prevents me from checking it too often.
My gmail emails that I use with app passwords etc. all had their prices raised recently to $7.20/month. For a long time I was paying $5. So it should still be cheaper to use Google over Proton for 1 domain, but if you have more than 1 maybe it's worth switching.
I haven't tried Proton as a developer yet (app passwords, using their APIs, etc.) just as a user. Overall it's a very similar experience to Google.