I think you have to be careful about reward systems. They can be useful to train or encourage behavior, but you want the behavior to become intrinsically motivated, not extrinsically motivated (only for the reward). Reward systems should have some sensible end to them (age, date, item/reward runs out, etc.) You also want to be careful about being too specific, or you can hinder "over performance" or set up a system that is appropriate for one kid and not another.
We used rewards for potty training (called it pay for poop

). Big bag of M&Ms for one kid, gummies for another in a jar. Kid got one for production. System ended when jar was empty.
We didn't ever pay for specific grades (unlike some friends' parents), but had a "good report card" end of quarter treat for each kid. They could choose a movie, dinner out, ice cream or something similar. System ended as kids gradually forgot to ask for the treat. Didn't really need it for motivation for 2 (and one didn't care very much), but it was a nice way to give them some approval beyond "great job".
Chores are either "we all pitch in as a family to keep the house running" or specific payment for specific extra tasks (like mowing.)