However bounty platforms dont work well for open source in my experience. At best the bounty for a given issue is a few hundred euros

Bounty source shows bounties for thousands of dollars, and it is a pretty bad platform, it does not even show the most funded issues, unlike rysolv which does.

But i think one of the biggest issues is that of a conversion rate, most users are simply not aware of that option (Or don't feel sufficiently motivated to do it), If there will be a link in the issue template and a link on the read me to the top funded issue and i think it will really make a big difference, most of thunderbird recent spike in revenue seems to be due to a higher conversation ratio.