I'm currently using my old watch box for storing 3 IEMs, it's rather small and they are made of cardboard. Could doing this damage them? My average room temperature is around 15 degrees and it's a relatively humid place (I already put silica gel tho)

Yes, search up the various other bigger drawer boxes, watch boxes, earrings boxes, accessories boxes, et cetera that some other people use for their IEMs. There's pictures here on /r/headphones, and also Head-Fi, etc.

There's also some good earbud cases from AliExpress for like $5-10, lol. But there's always the $5-10 cases from Amazon too. Those will work just fine for individual IEMs/TWS earbuds/etc.

For humidity, maybe get those tiny Ziploc/plastic bags and put them in there, maybe don't worry about static/etc. Don't worry too much about humidity, just put your electronics away from the window during winter if you don't have HVAC or constant heating.

How to maybe soundproof your room for better noise reduction/isolation: https://www.reddit.com/r/headphones/comments/10epr17/struggling_to_enjoy_my_headphones_in_noisy/j4t4qaw/

If you get (legit, the expensive heavy ones, not the light/cheap/etc. ones) blackout curtains and tape them, it should do some changes. Though actually even using those plastic film thermal insulation kits did nicely as the vacuum seal on the window reduced the sound a lot.

Those thermal insulation kits will help a lot with not needing to run the air conditioner (or heater) all the time, you can keep it on a timer if your AC unit has that option. Just make sure to achieve that good/perfect vacuum seal with the windows when it comes to taping it. There's also those door draft stopper or like foam you put at the bottom of the door, those can actually help quite a bit too with retaining the heat/cold, believe it or not.

I used to store my Sundara in their original box. One day I opened them after 2 months of sitting and they had a really weird smell, it was coming from the earcups and the box itself. It was a weird smell but I put silica gel packets in it and the smell went away the next time I opened them. This should be fine.

Watch boxes are good for earbuds and cheap iems and cables. Very good alternative to the VanNuys cases. Once you get into the higher end though, I prefer one single small case per IEM/Cable. Smaller dimensions just do not work as well for the nicer andor semi rigid cables.

Which presage model, mate?

I forgot what it's called but it's white and there's snow like texture on it.

Thanks for the recommendation, will probably get a smaller watch case so I don't have to shove them together lol. Wish IEM nowadays comes with bigger case like those old Noble and Empires where you can fit many in one.

Lol. I store them in everywhere i can, rn some of them are in my gym bag =))

I prefer the Eletech cases for a single IEM/Cable. Some of the major makers, 64 audio, CA, etc, also sell their cases by themselves.