You can not have a voluntarily given constant amount benefit also those that didn't opt to give it. This is highly unstable. You either have to threaten with not being able to benefit if not paying or make the amount paid vary between how many people opted in.

It would also expand the circle that would have to be competent in state money handling to a very large circle. Part of the reason why having representatives is an upside is that most people make very similar decisions but if everybody needs to come up with them independently that is a lot of duplicate work. Rather have a couple persons from each of the qualitatively different traditions (so you don't miss a type of decision) have the same output with less work. With representateievs they can work full time to make quality decisions. Those that don't earn a living with it have their day job to hinder their abilty to contribute to the various negotiations.