Take a look at Portabella (https://portabella.io), we're not feature complete with Notion but making serious progress!
Am also keeping an eye on https://anytype.io but I still haven't been drawn for their closed alpha grrrr
My company is evaluating Notion as a replacement for Confluence mostly, though we are looking at it's Kanban features also.
We've just launched our Documents feature, aiming to be a G Docs/Confluence alternative. Still early days though!
Hopefully open source soon!
The reason for e2ee is so you don't have to trust the provider. If you're hosting stuff yourself why wouldn't you trust your own infra? As long as you've got a TLS connection to and from your servers + encryption at rest, client side encryption gives no particular benefits.