My solution was a custom utility bed so it still has bed rails, but height is 50 inches so no problem leveling the fiver. Flatbeds are heavy because they are intended to carry heavy weight items anywhere on the bed, but pin weight is very localized. My utility bed only adds 400 lbs.
Cost was significant, around $5k with upgrade paint and clear coat. But that was 14 years ago, 10 years on the previous truck and 4 years on this truck, it will outlast my need. On both trucks, I sold the factory bed, tailgate and bumper for around $2500 each. Cost of a used bed and bumper to sell the first truck was less than $1000. Even with misc costs my net cost is about $100/year.