They just don't like cage cleans
They find it quite stressful. There are a number of tips on here for avoiding cage clean stress. It's difficult with a small cage(new build one sounds great!) but if you have deep enough substrate, you can just "spot clean" mainly rather than full cage cleans. Particularly if they use a litter tray. Then you just empty the litter tray every 5 days or so and the rest of the cage stays clean - they tend to pee in wheels but the wheel can be cleaned separately as and when.
I always leave the nest and the hoard alone unless it is pee'd in. They are very particular and anxious about their nest and their hoard. She may be digging in her house during the clean,to try and hide her hoard so you don't steal it! If the hoard is pee'd on and has to be removed, you need to put new food back in exactly the same place. It's a territorial instinct and if they think an invader is going to steal the hoard they can start peeing on it to deter invaders - so it becomes a vicious circle then. In fact if big clean outs are done too often, they tend to pee on everything sometimes to "scent mark" it.
Pet shops tend to recommend weekly full cleans, but it isn't necessary. Poops are fine unless they start taking over - they aren't smelly or dirty really. They are like little black seeds and they hoard or eat them sometimes for extra vitamins (which is normal).
The method I use is - litter tray and spot cleaning - and with a large cage that can mean the substrate doesn't need changing for about 3 months. Clean wheel a different week from the substrate, and clean any toys a different week again. That way something always smells familiar. Even when doing a substrate change, you can replace the bottom or clean third and mix it in so it smells familiar.
They are funny little things but if you remove all their scent from the wheel regularly they may ignore it! So adding a treat tempts them back in. I tend to accept the wheel gets a bit pee'd on and not over clean it but just occasionally (depending on how bad it is).
Difficult when it's hard to get hold of substrate. I believe corn cob isn't recommended either. It should be fine for now. Glad you've found some aspen!
Do you know about Zooplus? They have an Italian website. They are based in Germany but ship all over Europe and have a good selection of substrates. If you buy in bulk the postage isn't so much of an issue.
What you could do is just tear up a whole bunch of plain white toilet paper into strips and add that to the cage and mix it in with the current substrate. It's good for nest building, so always good to keep putting a pile of strips of tissue/toilet paper out for them to take and build/line the nest with even if you end up using a different substrate. It's safe if chewed or swallowed (apologies if you already know all that) and they seem to like paper strips for nests.
Zooplus seem to sell different products in different countries, and a lot of the bedding listed for Italy mostly isn't suitable for hamsters - but they do sell Carefresh in large packs. The safebed paper bedding is ok but it's in a very small pack so would work out expensive. You could get Carefresh, Hemp,
Lettiera Carefresh Natural | zooplus
Lettiera di canapa Hugro | zooplus
This Chipsi softwood bedding is a German brand and they have quite strict kiln drying regulations so it's a good one. It's part pine and part spruce - which is better than all pine because spruce is equivalent to hardwood in terms of phenols.
Lettiera Chipsi Super | zooplus
Hope that helps!