Getting a YumBox was a game-changer for my kid's lunchboxes and the compartments made it easy to do a formula of a lunch when half asleep. The boxes are pricey but *excellent*. We have the original kind:
https://yumboxlunch.com/collections/yumbox-original
No idea if this is at all helpful, and kid is 'only' vegetarian...
Section 1: always veg, usually a couple. Eg: carrot sticks, cucumber, gherkins, beetroot, mini corncobs, mange tout...
Section 2: always "the main thing",. Eg: sandwich, roll, wrap, pasta, rice, couscous, crackers, sausage roll, cold quesadilla, yanno...
Section 3: always "an protein": Eg: lump o' cheese, lump o' quorn, wee sausage, wee quorn scotch egg, a nugget, handful of roasted chickpeas...
Section 4: always crispy. Half a packet of crisps (very seldom actual potato chips, usually hula hoops/ pea snacks, pop chips, sunbites etc)
Section 5: always fruit. Eg: easy peeler orange, grapes, kiwi (cut up + spork), strawbs, whatever doesn't get mullered in transit or doesn't go brown.
The sections are p small, so it's very easy to have variety and be using up really small amounts of leftovers for some of it.
For sandwich fillings, I struggled too... Usually leaves + hummus/coleslaw/'ham'/cheese and the like.
I was often deal jealous