I think I'd handle my bunker differently. "If the devil wants in, he'll get in."
Make a bad ass enough spell caster to actually protect himself a few rounds. Fill your sanctum with offensive monsters/guardians also. Then make the wards keep foes in. Damage them when they try to leave via wards. Compell hopelessness or fear. Trap them in a mental maze. Kill them at your leisure.
No one expects to be kept in.
Making friends with a holy roller to help keep your threshhold high could be a nice help too.
-----------------------------------------------------------------------------------------------------
Getting into a sanctum past wards isn't as hard as it sounds.
I assume most wizards breathe. Burn fires outside long enough, the heat will diminsh oxygen inside slowly. Burn long enought thats a problem. Could just super heat the externals.
Lay siege. Patience is a virtue for a hunter.
Send innocents there to get fried (in the case of Harry's wards). This works fine in some cases for bad guys too. bad publicity and mortal attention will make a wizards life hell. Enough people die or disapear in an urban area people will notice. Even in rural settings this works...just takes longer.
Convince local criminals to use the property for nefarious dealings. They die, wizard gains enmity of local criminals. Get city hall to build a road through the area, or something similar.
I prefer anhydrous immonia for those underground dwellers. It's heavier than air...its also very cold and poisonous. It will freeze dry your lungs fresh out of a tank. It doesn't get much nicer later. Pump that into a sanctum through minions or force magic (won't work for wizards who hex everything, but plenty of magic doesn't cause hexing. pump in the gas. They'll leave, come out, or die. Anhydrous immonia may not be cheap, but it isn't expensive.
Stop up ventilation systems or ways air gets in. Suffocation is a bad way to die. Similarly, bury the sanctum.
Sounds likely get in. Psychological warfare works here, play ACDC at all hours full blast.
Drop 1000 gallon propane tanks from planes (similar to bombing runs and if you can affor that you might as well use bombs, but the propane tanks will cause the inhabitants ears to bleed and maybe rupture ear drums better than traditional bombs.
All else fails.
Out of spite continually pelt their home with compost or carrion... manure and fertilizer; etc. No one likes a smelly house.
None of this is fool proof, but most wizards won't expect these methods of getting at them.