So I am basically trying to make a cut slit, but not through the material, but only in it (think of this as a slot to mount an acrylic piece for display in).
I figure I can do a solid rectangle, then just keep playing with the speed and intensity till I get the proper depth; but is there a better way?
That’s pretty much the only way to do it. Each wood is going to burn differently.
One thing that can help is to scrape the resins out between passes - there can be a lot deposited on the front edge, and it won’t burn away evenly unless you do. (I use a small chisel.)
This tries many settings on a small bit of the wood. From that you can make a good guess what settings you can use.