On Android, NoteLynX Pro has some neat ideas, including an outliner item having multiple parents, http://astrodean.blogspot.com/?m=1 Mindscope on iOS also allows an item to be soft-linked into multiple trees/parents for display, with a navigation path back to the original context.

ConnectedText on Windows, DevonThink and Scrivener on macOS have partial overlap.

For mobile: could doogiePIM import OPML files with dedupe? Many existing mobile apps can export as OPML. Could serve as a one-way sync/import.

For archival resilience: will the database format be documented, or could SQLite or XML export be supported? It's tempting to consolidate data into one app for UX/power, but we all know the long term risks.

For mail import, might be worth confirming that MBX import works from https://www.mailstore.com/en/products/mailstore-home/, a free (for individuals) German app that can archive email from many cloud services. Presumably doogiePIM stores email in a standard format.