This could be solved by the app developers paying Reddit a portion of what they collect from the users. If Reddit wants $5/mo for each third-party app user, then the app developers could charge what they want above that amount. e.g., Apollo could charge $7.99/mo, of which Reddit gets $5.00 and Apollo gets $2.99 minus Apple’s App Store cut. Different developers could charge different amounts based on the value they provide, but Reddit would get the same amount per user from every app developer.
Why would it require an increase in the workforce? Automatic provisioning and charging monthly per API key isn’t rocket science. I’m sure they have enough engineers to tackle this if the revenue opportunity is significant.