I think if you really want to sell it, you're right, people aren't biting in the mid 20s. I think $18,500 to $22,500 is the sweet spot here.
Some sellers want to treat these cars as if they were "collectible" and not subject to the rules of depreciation, but this is just not so. These cars are too new to be collected, and too many of them are for sale now anyway. Besides all that, the brand-new Mustangs keep getting better and better and faster and faster.
So I'd sell at a lower price, sure. It's not going to get better as time goes on for you.