Ideally I would store it in a wooden box at room temperature but leather can also get moldy so ideally at 55% humidity.
I have a medium sized collection of vintage film cameras and I store those in a glass cabinet between 35% to 45%.
This is a little dry for vintage leather so I store that stuff in a wooden draw.
You could use a
gentle leather conditioner but be careful and test it on an inconspicuous area first.
This is worth a look. There's a product called "Elephant Leather Preserver" that I use on vintage camera straps and cases.
Fully synthetic and German made and very reasonably priced IMO.
There is another product that is used to preserve leather goods in a museum.