How hard should a drawknife be?

Yes , you need a oil stone or a Water stone or a diamond stone in the 2000 to 8000 range. Heard of some folks using sand paper but never seen or even considered trying it:) Suppose if the blade is not to bad could strop it with some compound, but if you took a file to it way too late? A spoke shave blade should be able to shave the hair off your arm!
For what you want to do perhaps a draw knife would work better? Still needs to be sharp!

-- Lifting one end of the plank.

Sharpen first. Then, don’t forget that draw knives can be used bevel up or down depending in the cut(s) required. Angled blades cut a bit better, so don’t forget that. Makes for slicing motion. Make sure that grain direction is in your favor. Trying to cut on rising grain is a no-no. Leads to tear out.
Did I say SHARPEN?

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It depends on your file but, most likely. If a file marked it, it would likely be too soft to hold an edge. Just at a guess, I’d say anything you marked with a typical file would be <50-55 Rc. For a blade, you want to be upwards of that around 60 Rc.

-- I collect hobbies. There is no sense in limiting yourself (Don W) - - - - - - - - Kenny in SW VA

During the past 18 years of raising chickens, not once has a chicken complained about splinters. If a bird decided to have an attitude, he just simmered a bit with some wine and vegetables. Delightfully good! Or slow roast.