“First, put in an underground fire shelter about 20 feet down”
No need for 20 feet. A simple shelter just a few feet under will work just fine. Just get past the root layers where fires burns underground. Probably 4-6 feet in most areas.
Many fire bunkers can cost less than $20k, and some I’ve seen are about $10k, installed.
We don’t have a fire shelter because we have 5 different roads out of here and can vacate quickly.
What do you do for air in an underground shelter when the fire flashes overhead and it’s 1,000F just above you?