Holy moly is the marble thing a good system.
Basically the way it works is one marble is worth 15 minutes of screen time. When the kids do the chores they are supposed to be doing anyway, they earn marbles to put in their jars. When they are ready to use screens they set a timer and "pay" for it by taking marbles out of their jars. It's not so much a reward system, but a way of helping them balance out their time better between what they should be doing and stuff they know they need to monitor better.
I talked to them about it first to see if they were interested. Quinn (who likes collecting things) and Mona (who already does her chores without being reminded) were excited about the idea. Aden was wary, but agreed to give it a go.