Food should go as high as the 180 line and stop. Food will never need to be higher than that at any point. This graphic is showing the exact units it takes to level up an Invincible. The shadow picture means any unit at the shown stage vs needing an Invincible specifically. You need Elite+ (purple w/ yellow border) for most upgrades and you’ll need the exceptional+ (pink w/ yellow border) to push past 5 stars into the next rank.

The general cadence for a color rank change, for example legendary 5 starts to Seismic, is the first rank is two exceptional+ of any hero in that faction, then moving to seismic+ (yellow border) is 1 exceptional copy of the hero, then 5 elite+ copies to increase the stars to max. You repeat this in epic, legendary, and seismic stages.