I would seem to me that you could have a middle-ground between the teenager who will fiddle with his arduino for a few weeks (and get the free service), and the professional setting up monitoring on a chain of restaurants.
I’m guessing the REST-API users are 99% just fetching data to use or present in a more useful way than looking at the couple of graphs on the website or downloading a csv. And then once in a while sending a command to control something. All the REST-API functions for adding/listing rules, devices, users, … and all the fancy functionality of the OAuth system would get little use. That is for the middle-user, not the chain-of-restaurants user.
So perhaps you could charge a fee for just that – rather than pursue the corporate contract for a full system.
You can get nearly free databases from Amazon, Microsoft, Google or just set it all up on a $20 Raspberry PI and skip the internet entirely.
I don’t think I fit into the “chain of restaurants” model.