An item returned at 4pm is not available at 4pm. It needs inspecting, cleaning, charging, or refuelling first. If your system does not know that, it will happily promise the item to the next customer at 4:15.
Buffer time belongs to the asset, not the booking — a drone needs its batteries charged regardless of who rented it. Set it once per item type and let availability account for it automatically.
The payoff is that your availability becomes something you can trust in front of a customer, rather than a number you mentally adjust every time you quote.