Is there a workaround to pay for several years (stock up) with in-app annual subscription?
I don’t think you get any sort of hidden features with subscriptions through Apple. Since this is a service, it’s not baked into any OS upgrade or app, it’s just logic on how the system works.
I would reach out to the developer if you need this, but I would caution you - how could Apple offer a refund for something they might not even be able to deliver in 3 years. If the developer offers a side registration process - they an do whatever they please, but Apple has been pretty set on the terms for a while - so anything could change, but I would expect them to be selling annual passes at the longest term for as long as they’re on the hook to provide refunds when a purchase has issues.
I think you have a very good grasp - there’s no hidden workarounds here - it’s like asking a bank to change how they grant interest on certificates of deposit. It’s a pretty standard transaction from a mechanism what happens at the start and end - you’re just wanting them to change the term for you and a specific developer.