Suspension

You loaded the trailer incorrectly. The load should be further back, over the trailer's axle, so the weight is on the trailer's suspension, not on the tongue.

There should be some load on the tongue but only about 10% of the total trailer weight. At 10% of the weight on the tongue, the load on the hitch would only have been around 100 pounds, and the car would sit properly. I have a trailer for my Prius C that I use weekly, and I have never had a problem with the rear suspension on the car sitting too low.

 

Just an FYI; several people with Prius C here had to replace their rear structural independent suspension frame because of bad or broken wields. Tire was pitched, excessive toe- in or out.

something to watch out for....

 

You don't need a stiffer suspension. If the trailer had been loaded properly the load on the hitch would have been one hundred pounds, which would be no more than the added weight on the rear wheels of a single person in the rear seat of the car. Even loading the trailer to 2,000 pounds would only put 200 pounds of additional weight on the rear wheels and that wouldn't cause a problem. Looking at the photo of your car and trailer you probably had 400 pounds of weight on the hall hitch.

 

Was the damage to the car's frame because pulling a trailer or just because of rust or accident damage. I just looked at the frame on my PriusC and saw no problems despite pulling a trailer at least once a week for four years. Tire wear has been excellent.

 

That wasn't mentioned, but there was broken wield joints and rust involved. Not too common, but I'm sure towing would help this issue along.

 

they do make spings for that.
you can also get air bags for the rear. I plan on getting a set.
https://www.etrailer.com/Vehicle-Suspension/Firestone/F4136.html?hhyear=2016&hhmake=Toyota&hhmodel=Prius+c

.....I should check my suspension frame