duckduckgo is one of the few search engines that doesn’t require javascript to function.
Removes all the fancy stuff and leaves you with search results and nothing else.

Their site works fine without allowing javascript, that way it turns into quite a simple thing too!

SnappyMail seem to be a fork of Rainloop and both Rainloop and Snappymail appear to allow multiple providers - https://snappymail.eu/
Cypht seems to be a similar solution where you selfhost a webserver that acts as a web client to external email providers - https://www.cypht.org/documentation/
I find nothing about push notifications for either of those solutions though, and I’m not sure about how much the webclients cache.

A video where the uploader compares different phone operating systems using the LINDDUN threat model.