That’s one of the places I messed up. The original agreement was that they’d fund development of the features they needed, but I retained rights to the software and they got a license as long as they were funding development, but they could license a copy in perpetuity for a flat fee. We never updated the amount of that flat fee as the feature set grew.
I had thought I needed to develop a multitenant feature set in order to sell to more people with a subscription, but I was probably incorrect on that (in hindsight).
The bigger issue was that I didn’t know how to plan for anything like that. I was essentially in reactive mode and got way behind.