Welcome.
I see that you have read 2 hours worth of forum posts, so you are off to a good start. I’m sure you read that testing is the only sure way to dial in settings for non Proofgrade material. I doubt that any of us in the forum have been able to harvest the quantity of birch bark that you have. Users have, however, engraved on leaves, veneer, tissue paper, and cardstock. Your material will not be a significant challenge, but it will absolutely be different and you will have to experiment.
You will not start a fire because you won’t leave your Glowforge unattended.
You will be wise, in my opinion, to go through the Glowforge tutorials and get a basic understanding of speed, power, lpi before jumping into a new material. You will also be wise to read through the tips and tricks section of the forum and this compilation of information put together by @evans Common problems/questions