> its weighed in my balance by the fact that I'm contributing under the GPL
Are you sure you're contributing "under the GPL" though? It's common for such dual-license commercial projects to demand (lest contributions be ignored and not integrated) of external contributors that they agree to a Contributor License Agreement, which grants the dual-license commercial project the right to sublicense the contribution however they please… as opposed to being bound by the terms of the GPL like you would for their code. In the case of JUCE: https://cla.juce.com/
So it's really a totally asymetric situation:
* for their code: you either pay them a hefty price for a commercial license… or you only get it under GPL terms and thus have to release your own code using it under GPL as well (which effectively thwarts most monetization options for your work)
* for your contributed code: they don't pay you and (unlike the other way around) are NOT bound by the GPL but can sublicense your code however they please… and of course they make money with it, you don't.
Now I'm not saying that that would totally hold me off from contributing. It wouldn't bother me for small ad-hoc patches that I don't want to wait for them to fix. But it sure would be a disqualifying criterion for me when it comes to pouring in significant time of unpaid voluntary work. For that, I'd look for a more community-based project on an equal footing. But then again, to each their own criteria and evaluation.