Have you considered moving in the other direction? There's lots of work -- and value -- in bog-standard software with no mathematical/statistical/computational cleverness. Occasionally you can see an opportunity to fruitfully apply a mildly clever quantitative technique and see it actually get used -- but >95% of the time there's usually necessary grunt work to do.
How about trying contract work on enterprise software projects? E.g. pick a huge boring-sounding enterprise that has some weird legacy* software systems built for internal use. If you find the right one, they will have (1) users who use the system every day to do their job, and (2) lashings of technical debt, and they may be quite excited to have anyone on board to push forward efforts in "unit testing" or "continuous integration" or "implementing exception handling correctly".
inspiration:
https://www.butternotes.com/blog/catfish-programmers
http://edw519.posthaven.com/willie-sutton-would-be-an-enterp...
* legacy doesn't necessarily mean old -- maybe the system uses some trendy new tech stack, but no one wrote any tests, and the first wave of contractors already left
Your software isn't being used. That sucks. The feeling must suck.
A car maker who builds something for 0 drivers will be sad. A dentist with 0 clients will be sad. Etc, etc.
Go to a big company that has users, and see what that does!
Let's say Series C/D+ -- $100m+ raised.
If you want more users, focus on consumer companies. If you care more about the business making great profits, focus on B2B.
I agree that having no users has caused some or even most of your anguish and that needs to be fixed but I disagree that joining a large company with customers will necessarily make you that much happier in the long run. In the short-term it may help and perhaps it's something you should try but down the road you'll start to feel like an inconsequential small cog in a machine and possibly end up back at this state of mind again.
Such a change might bring a different type of stress. The feeling that you're helping lobotomize a generation with mindless entertainment instead of helping cure cancer. Also, the tickets.