I understand that this isn't going to answer your question, but maybe ask why you want a web ui over a local app.
I've gone through this too and when I thought about it, I realized:
- My phone has K9
- My tablet has K9
- My computer has Thunderbird
No other devices access my email or calendar, and a web mail client isn't really where I want to go. I originally wanted a web client so that I could access my email from my work computer when I was in the office, however it was too much of a security risk just to avoid picking up my phone.
I'll get Nextcloud going for this just to test, but I know that I likely won't use it much.