Not sure about this since too many missing details. What kind of wood, how big, what kind of epoxy, etc.
Based on what you have noted, the 850 could work well depending on experience level. I've used a planer to flatten a slab many times. Have a good straight edge, adjust depth control as you go along, take off the highs down to the lows to get flat. The 850 will get you close enough for sanding.
I would get as flat as possible prior to adding any epoxy. Depending on type of epoxy and heat reaction you could gum up the planer or any sandpaper.
You could of course make your own router jig or get one of the router slab flattening set ups woodpecker or others sell. This might also work better if wanting to flatten with the epoxy in place. I'd rather soak and clean a couple router bits rather than clean out an 850 or waste a bunch of sandpaper.