>so many glaring issues
like what?
like what?
And probably a million other things that don’t hold up today.
For server side filters I just set them up in Fastmail using the web UI. That's the type of action I do once or twice a year, so totally OK to hop on over to the web app for just that.
I have no idea what you mean by 'push notifications'. I have Thunderbird open on my desktop, and it shows me when there is email. I have K9 on my smartphone, and it shows me when there is email (I don't have it set up to display notifications, but that seems possible). That's basically all I need to do email.
Push notifications: https://github.com/freswa/dovecot-xaps-plugin