No real long term experience here - just over 12 months with pearly jaws in about 2" of sand. More would be ideal but they adapt to a more diagonal burrow in my tank. Blue spots are a bigger species and will require more depth (what you propose sounds right).
You could also attempt a PVC burrow - basically a circular pvc home with a T that you cover with a barnacle cover. I've seen this done for nano tanks with BS jawfish. I tried this with my yellow-heads but I think the "home" is too big for their liking.
In terms of CUC - I utilize strombus conches, cerith and nassarius and sea cukes to stir/aerate the sandbed.
I would advise the occasional vacuuming of a sandbed. In larger sized tanks a DSB might be self-sustainable with the right mixture of fauna (worms etc) but smaller sized tanks will require intervention (e.g. vacuuming or period renewal of CUC/etc).
just my 2 cents. Good luck - jawfish have fantastic personalities. Mine stay about 4 inches apart and once in a while share the same burrow.