I had to flatten my 3 1/2" thick hard maple work bench top a year ago. I was not confident of my hand planing skills so i only did the bottom with hand planes - the top I did with a router sled. Overall the sled might have been a bit flatter over the 8' length but the hand tools were faster start-to-finish. the only thing I would use a sled on for sure would be some type of live edge slab that i am sure I would hack up with hand tools.
Well, it's done. Flat both sides of both halves. Son in law and I took them to Kettle Moraine hardwood where I them put through their big sander. The eyes of the store's manager bugged out a little when I told him how big the pieces were, otherwise it was just another day at the office. $5 a minute. It cost me $40. And saved me a lot of time. fwiw, it was the stores employees that did the work. They don't let me back there with the tools.
An excellent solution, sounds like it was well worth the $40 investment.
~Garth