I wouldn't think your shrimp are eating the giant hygro, they aren't known for eating healthy plant growth. The holes are likely from a deficiency. Any dying tissue is relished though, so that might explain them picking at those spots. As for brands of foods, most of what I purchase
comes from the poster above me. Most of what I feed
however, comes from my backyard. I use a lot of oak leaves, which can be left in the tank until completely deteriorated. Lots of other leaves are safe too, that's just what grows where I live. For years I bought cattepa and Indian almond leaves and so forth marketed for shrimp, but as far as I can tell they offer nothing that oak doesn't. I will use tiny amounts of vegetables from my garden, usually blanched, but anything uneaten is removed at the end of the day with those.
While they are constantly picking at food, these things eat an almost inconsequential amount. If they don't swarm the food that I offer, I am of the belief that they shouldn't have been fed that day. There's a YouTube video that goes over their anatomy and when you see the size of the stomach on a shrimp, it offers some perspective.
My biggest advice is to just leave them alone most of the time and make every change gradual. Like many people I struggled with shrimp when I first started out. Now that I've tweaked things a bit and treat them less like fish, I have much more success. I don't use heaters, they are fed very little, water changes are small (10% max), and new water is slowly dripped back in the tank rather than poured. Fertilizers are kept to a minimum or not used at all in sensitive strains of shrimp. My hands go in the tanks as infrequently as possible. I think it's a benefit to provide as much oxygenation as you can (one reason I like cool temps). Shrimp tanks require a lot of patience and a different approach than we might be used to with fishkeeping. But if you've gone from 3 to 40, you're doing something right!