I think a ranking system might be easier. People are good at ranking priorities.

The idea breaks down for the rich who are being taxed the most, because nobody wants them to have any say.

You could maybe do it for some percentage of taxes. Perhaps only for things that are desirable but not necessities (maybe Symphonies, science, high arts funding, sports funding, humanities education, BBC, other things people think they shouldn't pay for).

Although that would make people ask for a slider to reduce their taxes (to zero, thank you).