There's a sticky and yellow deposit on the thread

All is good. It's organic beeswax. Our stuff is stitched with a beeswax coated thread called the Ritza Tiger thread.
It's factory waxed but we add an extra layer just before assembling a product.
It's the best protection against the elements, making it impossible to rip, and it smells good!
It will slowly dissolve in the fibers in a few weeks.

I can't put a lot of cards in the slots in my wallet

Worry not. New leather is always a bit stiffer. Like a brand new pair of boot it will need some time to "break in"
When a wallet is fully assembled, we do an "initial stretch" of every card slot and pocket with a wooden tool that looks like a thick rounded dull butter knife
You may only be able to put one or two cards at first, but, as weeks go by, you will be able to gradually increase the amount

I can buy a leather wallet for much cheaper at the mall

Definitely. Rolls-Royces are more expensive than Civics.