I copied diamond lapping film sheets from your post and pasted it into this search thing “google “ and it came up with an amazing amount of results!!!! Try it, lol just giving u grief but it should get ya what you need.
When it gets to that small It’s more polishing instead of cutting so no sure if diamonds are needed. My stones are diamond, yes but honestly, past my 1200 grit I just strop and done shines like crazy.