Trying to keep a failing mattress comfortable a little longer

I'll keep it short, I've got a Mattress 1st Elmhurst Plush mattress (king sized) for 6 years and it's been sagging for a while now. My wife and I are both warm sleepers so memory foam isn't ideal but having a soft feel was nice at first. Reading about this particular mattress, it's comfortable in the show room but reviews seem to mention a trend of discomfort after a few years. I've been reading about latex toppers and want to give a dunlop a try but because of the current sag in our mattress, is that viable? I'm unsure if because this mattress is a pillow top style, if you can flip it and put the topper on the "underside" so to speak. Reading about fiberglass risks I'm not sure I want to try the DIY plush removal and reinsert new topper technique either. Any advice is welcome.