Hi Tom,
I was doing some research on how to increase the security on my website and for some reason this thread came around.
One crazy idea, but have you ever considered setting up a Patreon page? In this way people can contribute according to their possibilities, even people who get the free version and then you can also set different perk levels. This could be something in top of the normal license fees and you can offer perks such as Q&A sessions, seeing you coding life, getting to know you and the team better, etc.
There is an enthusiastic community around GP and i think many of us would love to show our appreciation back!
Cheers,
Catalina.