Full disclosure, I am newish too.
I hate recommending people buy expensive equipment, but I just had this battle myself a few months ago.
That is an automatic top off system.
You say you've been having to work hard at maintaining salinity. It should't be very hard. With an ATO, it's downright easy.
I have no way of knowing what is ailing your Goni. You've had big swings in some of the major parameters (temp and salinity). I've been there too ;Bored
The approach I've seen a lot of successful reefers on here take is to start from where you are. We can speculate about what might have happened, or what might have triggered this or that, but we can never really know for sure.
With that in mind, you want to start eliminating variables. You've addressed temperature - check.
Next up is salinity. It's a big deal. When your salinity jumps up and down, so does almost every other parameter. Alkalinity, calcium, magnesium, and every trace element. They're essentially "watered down" when your salinity drops.
If you have, or see a tank with an ATO, you'll notice it makes minute freshwater additions several to a dozen times a day. Very small swings. Topping up with freshwater once a day is less than ideal, but it works. You start going 2,3,4 days, you're getting serious salinity swings.
If you prefer to keep topping up manually, be as precise as you can. Put a little piece of tape where your tank is "full." Get your salinity exactly where you want it, and only add freshwater to the tank to replenish any evaporation below that line.
On water change day, suck out saltwater, replace with new saltwater. Likewise with your canister maintenance. If you dump out saltwater, replace it with the same. Adjusting salinity in tank can and is done, but it's a pain and a risk, and finishes a distant second to maintaining stable salinity.
As to feeding.. I'm out of my depth there. I have a few Gonis and they're doing great, but my tank is 6 mo's old, so proves nothing. However, since being underfed is off the table, I'd say you run near zero risk of harm by not target feeding them for a week or two. Perhaps you're feeding them too aggressively or too frequently. Not feeding is just a way to eliminate that as a variable as well.
The only thing I can tell you fo-sho is that you won't regret getting an ATO