Since no one has recommended, I would recommend Notesnook. Open Source app with Encryption. They are also planning to open source the Sync (afaik) part too.
Another option is AnyType.
Another option is Notion (not open Source/encrypted).
Yes. I did mention APN (Apple Push Notifications) to Beeper Push Notifications which then routes via FCM for Android.
The whole setup does require active server cost + time to work on the app to bring it to parity with iMessage Features.
I think upstream might be the right word in this context.