Another idea I had is if in the tax choice idea, delegates could only fund certain things so that it's not possible for them to return tax money to those who vote for them. Then things can be added to or removed from the list of certain things that can be funded if it wins a vote.

As for the border protection argument, I don't see it as a big problem. Hopefully most things that representatives are allowed to fund would benefit the whole nation, as the ability to fund healthcare for an individual state would be unlikely to win a nation-wide vote. If everyone is benefiting the whole nation, it wouldn't matter that some states need to spend more on the military.