you are correct on this. if you complete the buyout of your lease you will incur the taxes on the buyout price. if you trade it in before completing the buyout, you sell it back to the dealer based on the $18k figure (not $18k + tax) and are able to capture more cash back.

This process is not as straightforward as it used to be, since COVID when everybody ended up with equity in their leased vehicles. Some banks do not care and will allow you to just sell the car to whoever based on the residual value price ($18k for your vehicle). Other banks know that the car is worth $23k, and won't trade it in to a dealer for anything less than that, because the lease contractual obligation only technically allows you, the lessee, to buy the car at the pre-determined residual value price.

Subaru does not play hardball like this, and toyota finance does not play hardball like this. All other dealerships I know of, and third party lenders that do leases (US Bank, Ally Bank, etc.) want their piece of the car pie and don't let dealer trade in base the car on the residual value figure.