Are you planning to air dry it? I believe the rule of thumb is 1 year per inch of thickness so you may have to wait a few extra years to get down to a reasonable moisture level unless you have access to a kiln or can build a solar kiln.
Other than that, I’m guessing it’ll be tough feeding it through a jointer or planer, so you’re probably better off using hand tools to flatten and true it. Chris Schwartz made a Roubo that way using 2 slabs of cherry.
Sounds like it’ll be a great bench in the end though. Best of luck
Old tree or emerald ash borer?