I agree with you - we definitly need a months property.
Also the prime/turbo info could be used for custom animations - maybe some cool prime smiley. It’s public in the chat anyway - so why should the api not provide this information?
On this note, they should also add a updated_at property, which is set to the sub/resub date - so you can request subscriptions ordered by updated_at, which is the most important request i personally have for this endpoint.