I bought a bunch of slabs a few years ago. Looked into building one of those router sleds to plane them after they dried. One thing led to another and I wound up building a machine that could handle 10’ long slabs.
I used an Arduino Uno running GRBL and a RPi to control it wirelessly. Would have been easier with a Jackpot board.
Much easier on the back watching the machine do the work instead of pushing the router back and forth all day.
ohh that’s awesome, I might ask some more questions about that! My back is dying flattening these slabs, adn I have a ton more to come
Ask away. I may not always reply right away, but I’ll get back to you eventually. There is no automated Z-Axis. I do that manually (adjust router height) before each pass. Typically take off 1/8" each pass, then do 1/16" on a final smoothing cut. Requires very little sanding when done.
Looks like a Super Beefed up LR2! Great build! Thanks for sharing!
I took a similar approach but completely manually and rigged together in about half an hour.
I cut 4 strips of plywood and nailed/glued them together to make L-brackets for the linear rails and then used 2 pieces of aluminium extrusion I had lying around with wooden cross braces. The acrylic router base I had fit nicely into the slots in the extrusion and made a good enough running surface.
Because the slab was too wide to reach across I cable tied a couple of long dowels to the router so that I could pull it from one side to the other slowly.
This was using a 25mm surfacing bit at the lowest speed on the router.
I would pull the router across to the side I was standing on, move the rails forward 15mm on that side, walk to the other side, move the rails forward 15mm on the other side then pull the router back across before moving the rails again. As above, the Z adjustment was done using the plunge stop on the router and adjusted down at about 2mm at a time. I think it took ~3 passes to completely clean up plus a finish skim pass. I think it was under an hour start to finish for the actual flattening, took longer to make the jig and set it up. Depending on how many you have to do, something like that might be an option?
Ended up with it being flat enough that I could barely see light under my 1.2m straight edge aside from on one corner to corner which had I think 0.3mm of dish to it. Plenty good enough for a table top!
Jeff - Be sure to seal the ends of your boards with end grain sealer to protect the wood from end checks and splits while it’s air drying. You’ll be glad you did.
Edit - never mind. Just noticed your post about painting the ends.
Aaaah, the living room, the perfect spot for a motorcycle and a 300cm long CNC. 
Just teasing, that looks pretty neat. 
Or dehumidifiers tented also work did a locust tree a few years back that way it is paer solar and part electrical
