The main reason I use pinned flat pedals is that I can adjust my foot position to alleviate knee pain.

On a bike, my knees seem naturally to find spacing that's wider than normal pedal width, so I use pedal extenders - the wider separation allows my knees to go straight up and down more naturally. If you go for an extender, my reco is to get something cheap to test but to switch to one(s) that do(es)n't rust if you like the effect.

Frankly, I am not a big fan of professional bike fitting for recreational riders, except when knee pain is a problem. A good fitter may solve the problem for you or allow you to avoid painful trial and error.