Hi there.
When mine ate little protein I switched breakfast around, maybe worth a try? Here I used to give cereal (baby muesli) at breakfast but changed to an omelet which at any other time of day he would refuse but the first day I tried it at breakfast he ate the lot, an entire egg. So here for a long time it was omelet every morning and at least then I knew he'd had a serving of protein and I didn't need to stress over it the rest of the day. It freed us up to try a variety of other meats and fish so he could either taste a bit or not and as time has gone on the exposure has been well worth while as he now eats some unexpected proteins.
I also used to make lots of bean burgers/fritters, just canned beans (no salt) drained and rinsed and mashed up a bit mixed with flour, egg, bit of left over veg and some sort of flavouring like herbs, spices or even a squirt of tom ketchup. They can be frozen after cooking.
Oddly it turned out mine would eat sprats and mussels, who would have though, so he gets protein that way too.
Peas are high protein so I would serve them daily if necessary! Peas, pea puree as a dip or a sandwich/toast spread, pea soup, pea burgers...
Pate and humus were also accepted here. I think sometimes it's just a case of experimenting within the limits you are willing to go to within your own family, for instance we like sprats and mussels I wouldn't have even though of trying DS with them otherwise but when it's your own meal you don't feel like you've made something especially and then it's wasted.
hope this triggers some ideas.