At that size if kept on the warm side 75+ and fed well once a month for 2-3 months is normal. Then the time line starts to stretch out some. She will skip a month, then 4 the 7 the 10 the be up to once a year. Not in that exact order ofcourse but that was just to give you an idea.
They are bulldozers all right! Mine had almost completely excavated the bottom of the mason jar I was keeping it in. There were two pillars of substrate holding up the entire surface, and the rest was a cave. Since I rehoused it, it seems content to just make hills and valleys.
This species LOVES to play in the dirt--I've never heard of them really burrowing, they seem to prefer to just dig a "scrape" like yours did. My little bug does the same.
Given the massively dark bum in that third picture, I'd say sometime in the next month it'll probably start refusing food and go into premolt. Probably sooner. Just be very careful--slings can sometimes start molting with little or no warning, and you do NOT want crickets in the enclosure when it begins.
I avoided the problem entirely by killing regular-sized crickets in the freezer and giving slings the carcass. They eat what they can, and the leftovers are easy to pick out, and can't chew on the sling.