One of the biggest rationales for using drywall is its fire retardant
capability. Bear this in mind as you search for a substitute.

Good point.

"Sisters on the drywall with drywall in between will make it more
difficult to ignite the 2x4 studs over being exposed. Perhaps not as
good a 1/2" over the studs but ...

One reason drywall resists fire is that the gypsum retails a heck of a
lot of water. The fire has to "dry out" the gypsum before it can get
much above the boiling temperature of water and that's WAY below what it
takes to set the wood on fire.

N.B.: "Fire Resistant" safes often use gypsum in the space between the
outer wall and the inner wall. Stuff a wall cavity with drywall sheets
would provide some fire protection.

ALSO, there are fire retardant paints that might be applied to the wood.

Like it or not, in a typical house/office the fuel usually comes from
the belongings of the owner/tenants. Books, paper, and most furniture
and office equipment can provide fuel.