When i'm working slabs I use a bigass
Makita power plane to flatten one face. I use a 6' level for a straight edge and a pair of winding sticks as long as the board is wide. I can go remarkably fast using this method, but then I've been doing it for 30+ years. Anything less than 12" wide I joint in house, and anything 24" or less I have done.
Then I take it down to our local production millwork shop and run it through their 54" wide planer / sander. It has a spiral cutterhead followed by 2 belts. It's scary fast for surfacing. Cost is $100 per hour, but it can process at least a dozen slabs in an hour, so the cost is less than $10 each.
This is a walnut table top being sanded. As I recall it was in the 42" wide range.
I've seen and used all the various router carriages and so on, but in my extensive experience you can not beat big guns when it comes to milling.
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charlesfreeborn.com