Whilst leaf litter will always dominate most terrestrial isopod diets, there are a lot of other things that can, and for many species should, go into their diet.
Protein
There are a huge range of sources of protein, but one thing to attempt is to provide protein that they would come across in the wild. For isopods that in the wild would come across streams and water bodies, food such as shrimp, and fish food that includes fish and fish meal are suitable.
For those that would not come across fish and similar it's worth looking at insect-based fish foods, there are a variety such as bug bites and fish science.
Don’t discount feeding them insects, insect byproducts, and other species of isopod. There is a concern among some people that feeding isopods to isopods is cannibalism, but in the same way dogs and cows are both mammals, but different species, there are different species of isopods. So you could consider culling feeder isopods from another healthy colony.
If you are feeding meat-based products make sure that it doesn't go mouldy or off before the isopods manage to eat it all.
One area of protein you might not have looked at is Bat Guano. Sold as a fertiliser, it has all the insect meal in it, already partly digested to make it the perfect food source for many cave-dwelling isopods, including many of the cubaris sp.
Guano
Guano is the waste material from birds, bats and other cave-dwelling species. It is so important as a fertiliser that literal wars have been fought over islands with rich deposits of guano. It is also the main source of nutrition for the ecosystems of many caves. With limited light, there aren't plants growing as the base of a food web. Instead, the nutrients are brought into the cave by bats or birds. Beneath their roosts, piles of guano build up. Many species of bat have poor digestion, meaning that their droppings contain undigested material.
In a similar way, many islands have bird guano as their main source of nutrients, adding the fertiliser that everything else grows on.
This means that for some isopods guano is an important part of their diet. But there are a couple of issues. Without knowing where the guano has come from you don’t know what it is made up of, and there is often a large quantity of ammonia in the droppings.
The way around this is to use it as a part of their diet, spreading a small amount out across the substrate, this allows bacteria and other microorganisms to convert the ammonia and ensures that if there is something that your isopods wouldn't see in the wild, they are not going to be able to eat a large amount.
Vegetable matter
Various veg can be fed to isopods depending on the species. But for an easy meal look at the various Repashy foods. Sold as a powder you mix it up into a gel. That can then be put in the fridge or even frozen to extend its shelf life if you mix up too much.
Two we love are Bug Burger, and Morning Wood. Made from the plants and wood that isopods would see in the wild, already processed to allow them to digest the food easier.
Micronutrients
The one thing that is easy to miss is the micronutrients that might be missing from a single area. Imagine if the soil in the area you collect your leaves from is depleted. That can be passed on in the diet. The advantage of adding in a variety of other foods means that if one area is deficient others will provide the missing part.
There are a number of supplements that you can add to your isopods diet. But a healthy diet will add most of this to their diet. As a creature that has evolved to live on the scraps and detritus from other living animals isopods have evolved to make the best of what they are offered.