Earlier quoted context omitted.
> It is waaaay more efficient for these to be fixed by an engineer working not in their spare time, but as their full-time job. You miss the possibility of simply offering to pay a developer with the relevant expertise to fix one of these bugs for you. I can tell you from experience that there are FLOSS developers out there with sufficient expertise and professionalism to do this.
Expect 500 euros per day for a cheap developer in Eastern Europe. Maybe similar for Indian outsourcing or Latin American. Expect over a 1000 dollars per day for a qualified developer in a first world location. Point being. No individual can pay that much and the develop should rather get a day job unless the software has high earning potential.
You could create a cooperative of users that hires one or two developers to work for all of them.