This looks great, well done. At a previous company we had a relatively simple billing structure, but it was unusual in that it was usage based + an honesty system (clients could elect to not pay in any given instance if the product didn't add value in that instance). Add to that, the client often wouldn't know whether the product added value until weeks or months after the usage. We ended up writing our own billing stack, and would have loved something like this to just build our little quirks on top of.

And as you say, everyone kept telling us "Can't you just use Stripe or Chargebee?" but the answer was always no.

I will definitely check this out next time I'm setting up a billing system!