I’ve gotten so used to the web app that I don’t find I have the need for a separate email client. If Proton released the web app as a standalone email client that would be a different story
It exist Thunderbird Extensions for adding a Quick Access to Proton Mail and Proton Calendar. It would be nice the make a Thunderbird Fork or create a Thunderbird Plugin as Bridge.
Does the Web app work offline at all? I use thunderbird currently partly for this but also partly for my calendar which I'm still transitioning from self hosted caldev to proton calendar