Have you explored the Search URL parameters at all? For at least some of your requirements you may get some benefit. For example &order_by=updated_at would (I imagine though I haven’t tried it) get any updated obs to the front of your list.
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-1-of-2/63
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-2-of-2/18792