The pellets are actually fine for them, and if you got the small rodent ones, they are nutritionally complete. You're doing fine with him! I offer a mixture because to me giving a forager one kind of food would be like me eating boneless skinless chicken breast with nothing on it for the rest of my life. The mouse may not even care if that's all he's ever seen, it just makes me feel mean

Regarding tarantulas, super easy pets to keep. They don't need much space, they don't stink or make noise, and are low maintenance/cost pets. They are content on just crickets, though you can feed very small vertebrates to the larger ones.

The only "downside" if you care about this is that it's not really going to do anything except sit in there looking cool. You can handle them, but there's no real bond or anything like that.