The old variety formula allowed the 30 berries diet. But the humble value of the current variety bonus allows to mostly ignore it (my assumption based on limited observation), continuing leveling on cheesy diets, although of a different kind: eat one-two berry and then spam the most efficient food you have, adjusting it to the momentary satiation situation.
I suggest introducing a kind of hardcap on the old variety formula, meaning let it work as it was before patch, but each kind of food you eat can't reduce your requirement for level-up further than N * FEP sum of that food, where N is some constant.
For example, let's say N = 4. Eating q10 dog rose hip, +0.1 STR FEP, can't reduce your FEP meter more than 0.1 * 4 = 0.4 points; if N * FEP sum > variety bonus according to old formula, the old formula applies.
I don't insist it's necessary better or is the panacea to all world problems, but I think
- it solves berry diet for advanced characters case
- and yet it retains the significance/fun of the variety bonus.
It would make progression uniformly easier than it is right now (for better or worse), still dealing with tactic addressed in the last patch.
More broadly the basic thought may be described as "If eating too much of primitive food was a problem, tie variety bonus with food value, making it still big deal for advanced food but limiting the possible effect for simple food."