I don't like flexy trailers, so I homebuilt my trailer. Flexiness isn't good for the trailer or boat, it's just a way for compensating for improper axle springs IMHO. My new trailer is fairly heavy and very stiff, but I used rubber torsion axles which are well matched to the loaded weight of the trailer.

The converted power boat trailer is probably way over-sprung for the weight of the 5.0.