As someone who spent five minutes writing a pinboard client for my own personal use [1], it was frustrating that there was no way of testing these kinds of cases since even a test account would require extra payment. If pinboard were to pivot to a more api-friendly service, having some kind of test account(s) would be high on my wishlist.

[1] Almost the only feature is to prefill things like title, description, etc. because I wasted so much time, when using pinboard, viewing source and copying these manually.

37000 should be fine. Try changing the device’s auto-lock setting to 5 mins and leave it undisturbed for that amount of time for the initial download to go through. I’ve got a few users with 100000 bookmarks.

This support article for Pins would give a bit more detail: https://get-pins.app/posts/pinboard-sync-v1/