Maybe you live somewhere that you can keep your hay in completely perfect temp and very low humidity, but most places if you keep the hay in a bag it gets moldy since hay still contains some moisture, otherwise it would crumble to dust. That is why on farms you don't see hay in sealed bags, it's baled and stacked in barns where it stays dry, with good air flow, and in the dark. Hay is good for up to 3 years after harvest, and that is sitting in a barn not sealed in an air tight bag. If it takes you longer then 3 years go through hay then you should buy smaller amounts. I buy most of my hay, but also grow some of my own and never put it in a sealed bag ever. I have some from last year that is still sitting in a box and my guys still go nuts for it and prefer it over a brand new bag of store bought hay that comes in a sealed plastic bag.
I don't feed pallets but I do feed pellets, I feed the oxbow essentials food, some other good quality foods are Mazuri and Science Select. Maybe I am misunderstanding you you are just mixing different brands of pellet only foods? I thought by "mix" you mean thing with the little bits of junk food (fruit, veggies, nuts, seeds, etc) mixed in, those are what I am saying are inappropriate for chinchillas. Of course chins like them but just because they like them doesn't mean they are healthy, most kids like McDonald's, does that mean they can eat that every meal and live a long healthy life? Most companies don't actually research proper chinchilla diet, yes they look at what chins will eat, but mostly are going off old info for chins, are looking at rodents in general, or even lump chins with rabbit and guinea pigs when looking at diet since they are similar and there is more info available on them. I guess maybe I'm hard to please, because I'd rather my chins live a long healthy life rather then have them die from preventable things at a young age. Fruits are too high in sugar and can lead to diabetes, obesity, liver and kidney failure, tooth decay, among other things. Vegetables are also high in natural sugar and can also lead to gas, bloat, and diarrhea. Seeds and nuts are both high in fats which can cause obesity, as well as fatty liver diseases. A lot of those issues happen over time, so you may not notice them for several years. Lastly, since chins are herbivores, not carnivores or even omnivores like most rodents, they can't digest animal products. Maybe you are referring to different treats, but some of the treats I see people commonly asking about in the US and Canada contain some or all of those things. It also seems some pet stores also claim chins only live 5-10 years, and yes, if fed those kinds of things in their diet most would only live past 10. Obviously there are always exceptions, just like a human eating junk food everyday and still living into their 80s or 90s that doesn't mean that is healthy. With good genetics, proper care, and proper diet chins have been known to live into their 20s the oldest I know of was nearly 30 years old, so chins dying at only 10 is like humans only live till middle age.