I used vetafarm mouse and rat mix whilst in Australia. In the U.S. my partner kept rats on Kaytee lab blocks. I also regularly fed leafy green veg. I used a oaten hay mix in the bottom of the enclosures in combination with compressed recycled paper pellets. They like foraging through it. I also fed live crickets as a treat time to time. It is amazing to watch them hunt crickets.
Basically whenever I cooked dinner I would give him or them a few scraps from prep, a small piece of veg or lettuce. I also would give a small piece of hard boiled egg. Don't go over board on animal proteins I only gave egg or insects occasionally. It wasn't a regular offering. Just think of how mice live. Normally they forage for seeds, grains etc and once in a while they bump into an insect or egg etc.
So do your best to replicate that. They like climbing narrow sticks to forage grains, as this is the purpose of their tail. They are very good at balancing on a bouncing plant stalk.. it is amazing!
My best advice is to keep them interested in exploring and foraging. They are very clever. Also as you know, a clean house is SUPER important to protect their sensitive lungs from ammonia complications. No Pine shavings or high dust and dander bedding. Mice are also very sensitive to sudden temp changes, a cold night can cause an elderly mouse to perish.
They are really rewarding pets. Very much like building trust with a bird, they need to do things at their pace and trust you. It sounds like you have already made good progress with trust since Shredder is already hand friendly
