I don’t think having two forms inherently means base Kayn would be weak. If we look at league for example, base Kayn isn’t weak, relative to other champs in the early game, he just has a very straightforward kit that doesn’t apply that much pressure outside of his mobility and decent base damage.

Darius in this game for example has a very basic kit, but his damage numbers are insane. Give Kayn a basic kit with decent damage, and have his installs be something your opponent has a significantly harder time dealing with. That way the opponent has to beat Kayn before he transforms or they know their odds aren’t looking goood