Ultimately I've learned that this is not a good enough problem to turn into any kind of real, money making business. The problem isn't salient enough to really convince enough people to part with their money.
I'd rather have a product that makes no money, but gets wide usage, than have a product that makes $50/month with 2 users. Which is why I've decided to abandon a SaaS model and revert back to free.
But it doesn't end there. I was able to negotiate with the subscribing business customer for them to keep paying $15/month, which is enough to cover the site costs. They agree to keep paying that, even if the site is free in general, which means I can sustainably run a free, useful web product. If I look back to the beginning of this project, because I had even considered monetization, this was the reach goal. Have the site be free, and if I can find some way to break even, all the better. And that's exactly what I've done.
It's been a fun and interesting trip. Despite being back to where I've started, the process of monetizing a web app, implementing payments, selling a contract, etc. has taught me so much.
This is the state I expect to leave the project in for some time while I go off and explore other things. I'm happy that I've found a way to have a happy ending.