You are 100% correct his early game is really weak, which is the reason why ancient coin is a great starting as you have less chance of being poked when acquiring gold.
I agree with the fact he has no main rune for the start which can be hard to lane with and Guardian does benefit him and it is viable, but I personally feel it isn’t really needed as engagements isn’t the best idea before taking the form of Rhast. Of course there will be ganks and skirmishes, but getting something out of them can be hard so just keeping away from them until Rhast with aftershock, is in my opinion the way to go. With regards of Kayn having problems getting Rhast early vs ranged opponents, yes it is true, range and being able to engage is a little harder as Rhast, but can be much more Rewarding and does require combos as he is more built for brawling compared to SA with the extra range and mobility. These combos may consist of using bushes to get closer to your opponent and using your E (which speeds you up) and Righteous Glory at the same time to use the element of surprise against your opponent. Also using your E just for speed can also be better than trying to gank/engage through walls as the indicator can easily give you away and ruin your attempt to engage.

Thank you for commenting and sharing your thoughts as you have allowed me to gain an extra insight on Kayn support, if you may answer this question it would be greatly appreciated , have you played Kayn support and do you think it is worth continuing to play?

Feel free to continue sharing your thoughts and ideas and if u have any questions feel free to ask!

Cheers,
NoHard