There’s a lot of good leather info on here, try reading through @jae’s posts.
https://community.glowforge.com/search?q=%40jae%20leather%20in%3Afirst
and hmm, @morganstanfield is also a leather expert, though she hasn’t been active in a while.
Search results for '@morganstanfield leather in:first' - Glowforge Owners Forum
Good paces to get started.
(and generally speaking dyeing your leather first presents no problem to the laser, it doesn’t increase toxicity unless the dye itself is toxic, which is not really a common thing. All things you laser will be toxic to some degree, this is why we have good ventilation or filter setups. Get your vent in order and your only real worries are fires and other ways to damage the machine. Read about PVC and why you definitely don’t want to laser that.)
You seem fairly new to the forum, you might also like this post, it helps answer a lot of common stuff: