I use Paralino - the app is based in Austria and features full E2EE by default. The app even lets you view the encryted data on the server from the settings page. There's a full section of the website that includes the tech flow for account creation, encryption, groups, etc. It's really detailed. Also has a super clean UI, and the dev is open-sourcing the app code in the next couple months. it uses Google to serve the map tiles by default, but the dev is adding OpenStreetMap support in a future update. Play services are only required for non-delayed push notifications, not for location sharing.
The only downside is if you want multiple groups, or more than 5 in a group, there is a subscription. It's the only way the dev can make an app this good.
(I'm not the dev just in case anyone's wondering, I've just reached out to him a ton over email. He uses Proton Mail for the paralino support address, which is also a good sign.)