I see your garage door is insulated, is the attic above the garage insulated as well? You said you were in the south, I’m in SW Florida and my garage would have RO and premixed saltwater near 90 degrees in the summer is why I ask. I wouldn’t trust that drain line/condensate line or whatever it is, just run the waste water to plants you want watered or just outside the garage and let it drain down the driveway. Amazing that you bought the house with the tank already there and you’re diving into saltwater head first. Hope you have deep pockets, nothing is cheap in this hobby as you’re finding out but I commend you for going all in.