How about just adding a small minisplit with heat. Warm day = use the minisplit. Hot day = existing central air. Very hot= use them both. If one goes down you still have something while you wait for repair. The mini split could also provide some heating when its mildly cold.
We had a mini split installed into our garage (a Daikin that’s been great). I’m not sure how we would do that for the house. It’s a forced air system with outlets and return air in every room. I’m not sure how feasible it would be to pipe the output from a split system into that ductwork.
Ductless mini-splits aren't ducted. (These are the type most are familiar with, the ones with a large air handler up on the wall or in a ceiling cassette.) However, mini-split just means that the condenser and evaporator units are separate ("split") and are smaller capacity ("mini") than a traditional central air system (which are also "split").
There are ducted mini-splits in addition to ductless mini-splits.
