Hi @pindelicato ,
Thanks for writing into the Community! Great to see it being active!
In my own opinion, I think you have the right idea. The Subscription Event itself would have the subscription_type for an annual or monthly subscription. You could have this as an event property OR have it as a user property in that event and then you can update it if it were to change in the future. I think subscripton_type as a user property is ideal, especially as you can look at the user composition chart on that property then.
Are there other properties that should be tied to those those track events or set on the user and accounts?
I think this is better discovered later as you implement but off the top of my head, you might also want to track revenue for the event, as well as any other user properties such as how they paid (card or cash, for example), which plan etc. This would depend on your overall use case but your subscription_type idea is right on the money! :)
I hope this helps! Let me know if I can help in any other way.
Kind Regards,
Denis