Maybe break up the vertical integration.
You either get to be a store or a distribution center, not both, also strip the trucking from them.
Basically letting them control the the entire industry gives them way too much power.
No need to buy the stores etc. Just do what was done to Telecom, strip out the lines business and make them sell the access to everyone.
In the case of the supermarkets; force the breakup of the distribution network. This also allows small local supermarkets to compete on price, because they get access to the same price that existing major stores have available.
How does a lettuce sell for ~$1 at the market garden and show up on shelves at above $6. It isn't just the stores, the internal costs from the distribution network are very high.
This is not a full plan, we need a full analysis of how costs are being added throughout chain from farm to supermarket, somewhere along the way big costs are being imposed.