I have found that I can handle the nuun up to a point (either heart rate or distance), then it just feels like fizz in my tummy. I have also used the "let it go flat" trick when preparing ahead of time. Sometimes I'll double up on tabs in my bottle and have an extra "just water" bottle so I can get more flat-nuun rather than having to add a tab in the middle of a ride and end up fizzy again.

Might switch to the endurolyte shake-up myself, too. My dad likes to put a lemon or dab of lemon juice in his bottle so that it's flavored water without being fully flavored, which might help with the taste; though I suspect after a while it won't matter.