It sounds to me like one way or another, you should get rid of your Sonata. You need one car, and now you have two cars, one of which (the newer one) now suits your needs better than the other. Maybe keep the Sonata around for a month or so as a hedge while you use the new car in case the new car proves to be a lemon, but why maintain two cars when you only need one?
I understand wanting to get your money's worth from a purchase, but it sounds like you already have in the case of the Sonata. Unless you literally want to drive the Sonata "until it dies," and likely get into an accident in the process, there has to be some tipping point where you switch over to a new(er) vehicle. And while you are out "getting another $1000 worth" out of your old car, your new(er) one is sitting there depreciating, and you are not getting your money's worth out of it. Perhaps I'm missing information, but this sounds like a sunk cost fallacy thinking situation.
If I'm understanding your post correctly, you got a quote on your Sonata from a dealer but not yet from Carmax, and have been okay with selling to Carmax in the past. So get a quote from Carmax too. Even if you ultimately sell the Sonata yourself, having more than one quote will give you additional information to work with. Maybe also look into seeing if you can donate the Sonata somewhere and get more than $1000 as a tax write-off, but I think that may be unlikely. Good luck!