Running Service to Outbuilding
Your electrician was thinking further ahead than you thought they were
As it turns out, your electrician did think of powering outbuildings when they put your 400A service in, after all.  The cheapest way to do a 400A service is to use a Class 320 socket feeding two 200A main panels; however, your setup is a bit more sophisticated and flexible, and that's a good thing in this circumstance.  In particular, you have a Class 320 meter-main outside, with two main breakers fitted and room for up to 4 secondary mains, under the old NEC 230.71 &quot;rule of six&quot; provision, feeding two 200A subpanels (which have main breakers of their own that serve as convenience disconnects) inside your house.
As a result of this, you do not have to worry about having the inside of the house opened up to install the feeder for your shed; your electrician can simply fit a suitable breaker in the meter-main's subsidiary section instead and call it a day.  They probably should move the spare breaker from slot 2 to slot 3 and add a QO1HT to clean up the minor situation with the existing spare breakers, or replace both spares with a QO215 or QO220 if there's nothing hooked to them to begin with, but that's really just a divot in the grand scheme of things.
That 400A service has plenty of space, too
Running a quick Article 220 calculation on your house, consisting of:

3200 ft2 at 3VA/ft2
7 small appliance branch circuit allowances at 1500VA each between the two kitchen countertop GFCI circuits, the laundry circuit, the two dishwashers, the microwave, and the garbage disposal
a 12kVA oven allowance (this number is kind of arbitrary, but there's so much excess capacity here that it doesn't matter in the end)
a 5kVA dryer allowance as per NEC 220.54
a 4.5kVA hot water heater
1/2HP sump and ejector pumps
a 1/3HP septic aerator pump
and a 27.8A compressor unit

gives us 195A of service load at the moment.  Obviously, this is easily within the capacity of a 400A service; even if your shed pulled a full 100A, something it certainly won't do given your description of the loads inside, you'd still have room to spare.  This also means that you don't have to downsize either interior panel in order to &quot;make room&quot; for the shed at all.
And that 100A figure is quite generous for your shed, too
Running another Article 220 load calculation on the shed, given that we're dealing with a 1500 ft2 shed here and two 1500VA allowances for garage door openers and shop tools, gives us a mere 4575VA, or 19A of feeder load at 240V.  As a result, I would have your electrician run a 1.5&quot; PVC conduit to the shed; that's a sufficiently sized conduit for a 100A feeder, but gives your electrician the option to run smaller wire now and let you (or the next owner) upgrade the circuit later if the need for more power arises, instead of grossly overprovisioning the wiring now just to avoid an expensive dig-up-and-replace job later.