Fair points on both, and the note field belonging on the transaction itself rather than mirrored into customer metafields is a reasonable critique of the workaround.
I’ve logged the note field gap as fresh product feedback with the team, and I’ve added your idempotency feedback to an existing tracked request alongside the sister thread. No timeline I can share, but both are now visible to the right team with your use case attached.
Thanks @ogasawarakyohei!