A sled and router would be the easy way to go.
I use hand planes because thats what I have and they work great, if you have sharp blades and the planes are fettled correctly.
A Stanley #5 and Stanley #7 would flatten them in about 10 minutes per side. Getting a #5 off flea bay and expecting it to work upon arrival is not realistic. Buying new ones (Stanley type) can be expensive with Woodriver on the low end and Lie Nielsen on the high end.
If the grain is not too crazy, I can get a finished surface from the planes. no or very light sanding required.
