For the 800 names that were missing declension data in the database, it seems like the most straightforward thing to do would be to assign their declensions by hand. It shouldn't take a native speaker more than a couple of hours (if some name they haven't seen before is ambiguous, then whatever they guess at least won't sound obviously wrong to other native speakers). Alternatively, very cheap to ask an LLM to do it.

Encoding them into a trie like this would still be a good way to distribute the result, but you don't have to rely on the trie also being a good way to guess the declensions.