Mine started to fly out of the brooder at two weeks! I'm sure you'll get this solved. Think like a weasel, too, any opening that would let a hot dog through...we have hoards of raccoons, foxes, bears, mink, fishers, bobcats, coyotes and our own feral cats.
I have heard of someone using metal stakes and hammering them into the ground every couple inches around the run... It should be easier than digging, but it might damage more tree roots.
I am sure that would get expensive though, unless you know of a construction sight or someone that works in construction that could get extra pieces of rebar and use those...
Your siding looks like the same thing we are using on our henhouse. it is OSB on one side and that pressed siding on the other. Do you know what it is called? Does it have to be painted?
And, yes, a predator can indeed get under the coop and into the run. We are sealing all the way around the base of the coop with hardware cloth, bent out a foot, and covered with dirt, large rocks, bricks and landmines.
I used garden pins to hold my hardware cloth down all around the runs. Worked great for us and grass grows up through it after a while so you can't even tell it's there.