Mastodon has the same fatal flaw. They want to keep your history and relationships hostage so you can’t leave.
You can migrate your relationships to a new Mastodon server.
And while you can’t directly transfer the history (the debate over how/whether to do this has gone on for literally years), you can export an archive you can keep locally, and there are tools out there to parse it and convert it to some other form (static website, whatever). Someone’s probably written an importer by now, though I’d have to look.
Yeah, it’s made more for refining and confirming details than for adding entirely new objects.
When I want to add something that’s not on the map yet, I either add a note in StreetComplete and come back to it later on my computer, or I add it using Vespucci. (I’ve been known to add items using Vespucci, then reload the data on StreetComplete so I can fill in details that aren’t in the presets and that I can’t remember how to tag manually!)