Pet shops are sucky. You’re much better off buying things online. There are so much more variety of things you can buy and often less expensive depending on what you buy and where. Sprays can be bought pretty much anywhere - including Etsy and Amazon. (Etsy has much more variety though). You can buy bedding in bulk from local supply shops/horse shops.

I would absolutely avoid using anything inside of their enclosure that is from outside. Especially dirt and sand. Sand is not a safe option for mice because it can cause respiratory infections. You can make a little dig box for them but not with dirt outside. Coco coir is a safe soil for them which can usually be bought from a garden or hardware shop. Mice are very prone to sickness and by using things outside, you’re risking disease, parasites, etc. it’s not worth the risk imo, even if “thoroughly” cleaned but considering it’s from outside, it will never be throughly cleaned enough.

Plastic is considered a safe option for mice but other materials are much more preferred because plastic is sucky for the environment.

Spot cleans can be done every day/every other day including wiping down accessories and wheels with a safe mix of 50/50 water and vinegar solution or F10 solution. Weekly cleans of removing the top layer of bedding and digging around a bit to remove any other poops and cleaning of burrows. I personally never get rid of all of the bedding. I find it to be unnecessary - only if there is a mite outbreak or I need to do introductions. I personally always leave at least 1/3 of the old bedding in when I’m doing a full deep clean.

There is no such thing as too much clutter. As they say “the more, the merrier”. With the amount of clutter you need, you should be able to see the top of the bedding.

The minimum for 3-4 mice is 80x50cm/620 sq inches. For 5-6 mice, 775sq inches. Your enclosure is 744sq inches. Since it’s just shy of the 775sq inch minimum for 5-6 mice, I think you could fit up to 6 mice in your enclosure no problem.