Here's a couple that I've thought of already
1- use benches (but wouldn't they tilt just as much as a chair? Or would this not matter because the benches wouldn't have backs?)
2- prop up the front legs of the chairs to make them even. Maybe use plywood and make tiers and cover them with the grass you can buy and then roll in to your lawn. (this seems like an exhausting amount of work and really expensive)