Agree you need a minimum wind speed just to steer and start the Serenity. Once going in motion with 2 knots of wind, no problems to glide on mirror-like water. In gusty wind conditions, I've found out it's better to lower the boom so you can stay hooked in all the time and get a better balance.
If you like outdoor work out on your Serenity, try her in no winds by slow and ample pumping of the sail. This is tricky because you need a bit of hull banking to keep her on track straight forward while the center of effort of your sail is moving upon each sail pumping.
As for supping the Serenity, this is doable but the round-bottomed hull is very unstable and tippy as compared to a dedicated racing flat-bottomed SUP hull. Never tried the K15 yet, sorry.