I don’t find rodents to go that deep, but it’s just not hard to add hardware mesh down, or out. Aprons work great, without having to dig down several feet. Maybe you can anchor through it deeper with mobile home anchors (or something along those lines) to prevent heaving?
I’ve got more of a rodent problem here than I’ve had anywhere else, I think due to forest and habitat loss and human encroachment. It sucks, because they’re all so trap savvy. Thankfully the barn set up makes it easy to keep them out of the feed room & hay, but they do try to move in against the barn foundation or under the run in shed.
Making aggressive exclusion the first line of defence with hardware mesh and thresholds and the like has been so much more effective than not excluding and trying to play catch up with stuff that’s already inside. And while it’s effort to set up initially, it’s less effort long term than checking traps and trying to get stuff out that’s already in.
(And, bonus, just keeping stuff out is overall more humane, and limits the draw to other animals, whether they’re coming to investigate a mouse in distress on a glue board, or an easy colony to pick from.)