Is flat rate the best way to describe it when there's actually a few different tiers and each one has hard coded rate limits?

Within each tier, each marginal token is an expense with no marginal revenue to offset. So yes. The platonic ideal for any subscription business model is zero usage.

I run an AI subscription business and we have our pricing set in a way that we make an acceptable profit even if all users were to max out their given usage

Of course. My point is that your profit still decreases as you approach max usage, ceteris parabis. It may be acceptable but it is less. Your costs are variable and your revenue is fixed (at least on a unit basis).

No, not for us because we have a lot of different tiers and so as their usage increases they buy bigger and bigger plans.