Maybe this is the kind of software that requires huge development costs. But maybe it would be worth 20 seats' worth of customers joining forces to fund a team of 5 people to build you a competing app tailored to your specific needs/wants and completely under your control.
Granted, that could bump your costs from $800K/year to $1.6M+/year. But only short-term. Once your software is production-ready, you drop the costs of your current software. So think of it more like going from $8M/10 years to $6-10M/10 years but having complete control to add the features you want. And perhaps having the opportunity to recoup $millions/year by licensing to others. Or, open source it and give others the same kind of control while benefiting from the features they add. Spread your development costs across more seats to further lower your $/seat.
Or, look at the 100 employees your vendor currently has and lose heart, then hope somebody with deep pockets funds a competitor.