The subscription tier is provided, it’s the sub_plan value. 1000 being $4.99 and Prime, 2000 being $9.99, and 3000 being $24.99
Their current sub streak, or the date of their initial subscription, is impossible to obtain with the current API. The subscribers endpoint does include a create_at field (and I’m talking about the subscription object here, not the user object contained within) but this field updates under certain circumstances, such as a change of payment method or change of tier.