They didn't have to use deadly force, at least initially. Tear gas and rubber bullets should suffice.

If it didn't, then storming the doors is when you use live rounds. As was done in one case (lady air force person pushing her way through the door or window (forget which)). Instead, one officer died from injuries sustained at the hands of the rioters. A few others died, also, though mostly of medical conditions (a heart attack or two, etc). In the following months a few LEOs committed suicide, and some people count them among the casualties of Jan 6.

But I think tear gas and rubber bullets would have thinned the crowd enough that there probably wouldn't have been any fatalities from violence. Restraint is a very good thing, but at some point you have to take the gloves off.