I would love to see some pictures of the setup, too.
I think I am with
@ElleDee here as an initial food for thought. That sounds like a lot of Prime (overdosing), and a known side-effect of overdosing with Prime is a higher risk of oxygen depletion. The fact that the fish are 'gasping' after a water change shows some correlation, but of course, it does not mean causation. Regardless, that is a problem to resolve. i.e. accurate dosing, and preferably, move away from prime and use something with Sodium Thiosulfate in it - which is harder to overdose on.
Unless the water supply is truly contaminated, then water changes (with correct dosing) should not have such a negative effect. Last weekend, I did a 75% water change (using API tap water conditioner on the replaced water only), and all fish are alive and well. The only aeration going into the new water is the pressure of the tap when filling up a 25L bucket - on top of good surface agitation within the aquarium. Apart from inhabitant stress, water changes of various sizes should not be a problem. An answer here (above changing your water conditioner and getting help checking water contamination) is to aerate your aquarium
hugely during the change. In fact, if we can see a picture, it would be great to see how much aeration is occurring with your tank, balanced with plants.
Also, something to think about is that you moved, and since then, fish have been dying after a water change. Does that mean it was fine when you installed and re-set the tank up in your new home, and then subsequent water changes started to see fish losses? Lastly, how did you transfer your filter media during the house move? Was there a chance it dried out or did not receive oxygen to keep the bacteria alive, and you have effectively started over again in a cycle? And in the process of regular water changes, you are never getting cycled?