Any decent payment processing service should come with enough tools to get you what you want. There aren’t many that are open source but a quick Google came up with results so they do exist.
They will allow you to track all of your active subscribers/donors and should allow you easy access to that data. Calculating and assigning voting power with that information should be rather easy.
The big issue with removing the headphones jack is just that it’s now impossible to use wired headphones while charging the phone.
For a lot of people that doesn’t matter but for some of us that’s a big deal. If they added a second USB-C port that would fix the issue.
But saying the 3.5 jack is legacy technology is also kinda wrong. A USB headset is not inherently better. You have to compare the digital audio converter that’s used. While USB headphones use their own dac, the jack uses the dac of the phone. So a cheap phone with high quality USB headphones will be better but a high quality phone with cheap USB headphones would be worse than using the jack.
Which even means jacks would be more sustainable because you only need one dac per phone rather than one per headphone.
And any form of wireless headphones are just inferior to wired connections.