2020 Crosstrek Base
You can't just weld steel to aluminum, you can friction weld it though. lolI don't need it removable, but I'm not sure that welding is the solution. The mating surface does not seem like it's completely flat, and I'm not sure how the steel would weld to aluminum. It seems like the potential failure is due to the thickness of the sheet metal.
Thinking about it some more, I wonder if you could remove the rear bumper and crash beam, and then access the tops of the holes through inside the frame. Then just use much larger washers to spread the load out more. That would also save me from needing to buy new hardware from eTrailer, which is $60. I paid $128 for the whole hitch the first time, so I'm not excited about that price.
You can remove the rear bumper then crash bar behind it and access that opening where the hardware sits. I personally didn't do it for my install, but some people don't like to enlarge the holes in their car, so they do it that way. I am sure you could hit up a tractor supply store or something like that and get some decent sized carriage bolt washers and a new bolt as well if you need it.
If I were in you situation, I would get some big rectangular ones if you can. That way it covers more surface area, I am guessing the ones that come with the Curt hitch are just the right size to not fail and to make install easy from under the car.