Many thanks for the kind words!
You raise good questions, but I'm afraid there are no easy answers to them at this time. Have we considered non-photo based sources? Speaking only for myself, I certainly have; specifically video or audio recordings, and text documents. Tropy's data model is designed, intentionally and at a high cost, as generic as possible in order to potentially allow for such extensions in the future. We also have a few interesting UI ideas with regard to non-photo sources which would be exciting to explore.
However, the fact is that Tropy was devised by a group of historians specifically for the work with archival photos so this remains our most important usage scenario and there are still quite a few features we have in mind to further improve work with photo sources before we are ready, I think, to consider non-photo sources.
Zotero and Tropy are being developed by an overlapping team so, naturally, we're thinking about how best to create synergies between them going forward. But again, speaking for the Tropy side of things, I think we're still a little caught up in working with photos and adding a few more elementary features before we can define new goals.
And, most importantly, Tropy is free and open source software, so that was just my opinion and I certainly don't have all the answers. All the work, feedback, and ideas contributed by the community here, on GitHub or Transifex is valuable input that will continue to shape the project and influence its direction one way or another.