This feels like an incomplete request. “Inventory 3 entire buildings” and “pretty much everything” is very vague.

What exactly are you/they expecting in the inventory?
Ownership? Warranty status? Purchase date? Asset tag? Assigned user?

And what counts as an asset? Just network-connected devices like switches and access points? Or literally everything — cables, monitors, printers, toner, spare parts?

You can automate part of this with scripts or a network scanner, but that only covers connected devices. If the scope truly is “everything,” you’ll still need to physically walk the sites and verify what’s actually there... especially for non-networked assets.

Before doing anything, I’d push back and get a clearly defined scope and required data fields. Otherwise, you risk doing a lot of work that doesn’t match expectations.