While the app is unrelated to music, it’s always nice when it can help discover one of my favourite composer!
If you import 40,000 notes, one issue you’ll run into is that the initial sync will take a while to complete. The best thing is to import everything in the desktop app, then start the synchronisation process, and let it run over night or as long as needed. It’s best not to close the app while it’s doing this (if you do, it will resume, but there’s always an overhead to start the sync process).
Once it’s all on the sync target, it’s faster to sync with new devices since it can download multiple notes in parallel. On mobile, you also have the option to not download attachments (images, etc.) by default. This can also speed up sync if you have many attachments.