I use anywhere from 3-5 inches. I currently have 5 inches of eco earth. My toads are burrowed for the winter. Have been since November.
As to giving them less substrate. I have only given them a inch or 2 before and they still tried to burrow it in. They would hunker down and close their eyes tight, as if they were completely burrowed. They have never been really fond of the hides I've given them. The only one they seem to actually ever use is a plastic basket that I cut a hole into and placed upside down.
I have also given them just paper towel. They shredded it trying to burrow and ended up all hunkered down in a corner. I feel too sorry for them and always give them deep substrate again.
Last winter my toad Jabba died. Sometimes animals die in brumation. There is risk, even when conditions seem perfect.
This winter has been pretty cold. But every time it warms up a little after a cold spell the toads come up and shed. I feed them and then they burrow again. They have done this twice so far. I have dug them up a couple of times to check on them. Probably not the best thing to do. But so far everyone seems to be doing well.
Last year I didn't dig anyone up or any time before that. But after Jabba died; like you, I was worried.
I'd only give them a couple of inches of substrate. Then they can somewhat burrow and you will still be able to find them easily. Also your toads might actually use their hides, unlike mine. The only reason that I give mine so much to burrow in, is because I am actually cooling them. I want them to have a little protection against the cold.