Digging down is difficult/near impossible in some areas. OP mentioned rocky ground, I have a ton of tree roots that run under the run space.Regarding digging down for hardware cloth; I hear that going down is better than going out. Can you let us know why out is better. Is it because it's recommended 12" out and you share 18 - 24? Sorry, I'm starting a new build soon and gleaning from any and all advice I can get. Thanks in advance!
Maintenance is also far easier on a horizontal apron as you can check the condition of it without having to dig it up.
I think in an absolute perfect set up you'd actually go out and down (diagonal) and go like 16-24", but the amount of work to do that (or the need for heavy equipment) makes it impractical for most people.