I think it would be quite good on flattish water.

A 75 yr old friend of mine has a Simon River K1 Laser. It is a K1 Masters. He bought it in Canada and used it there for a while and now paddles it in Australia.

I have the impression that Simon River knows how to build good kayaks.

Having chines is not the only way to get stability. One easy way is to have quite a bit of the length of the kayak close to the maximum waterline beam. In other words avoiding the very fine bow of a racing kayak.

A shallow U shaped hull is a sensible choice for a reasonably fast, reasonable stable recreational kayak.

At 21 ft it will be slower with one paddler than a 17 ft similar shape because of the extra wetted area, but should move along very nicely with two paddlers.

Some compromise or multi purpose designs don't work very well. I think this one would work.

If you do go ahead with the purchase, I would love to hear your report on how well it works.