Opening a permanent portal to an alternate dimension that resembles the past of our own dimension isn't really opening a portal to the past. It is opening a portal to a different dimension that resembles, but is not, our own.
This resembles "Off to be the Wizard" by Scott Mayer.
So it doesn't really affect our past. What the time travelers find is that whatever they do in the past, it has no affect on the future, since the past and the future are different dimensions.
Martin Banks is just a normal guy who has made an abnormal discovery: he can manipulate reality, thanks to reality being nothing more than a computer program. With every use of this ability, though, Martin finds his little “tweaks” have not escaped notice. Rather than face prosecution, he decides instead to travel back in time to the Middle Ages and pose as a wizard.
What could possibly go wrong?
An American hacker in King Arthur’s court, Martin must now train to become a full-fledged master of his powers, discover the truth behind the ancient wizard Merlin…and not, y’know, die or anything.
Off to Be the Wizard by Seth Meyer is my favorite comfort food book. It's very light, but engaging, and presents an intriguing premise that's fun to think about.
I listened to the audio versions of the series read by Luke Daniels, who was a great match for the material, I thought.
I want to read print if I'm sitting and relaxing, but if I have to be up and active, doing tasks like laundry or driving that leave my mind free to ruminate, audio books really save me from myself.