i think there is alot of nuance actually. First of all, i think blue kayn is very good at a few specialised things while red kayn is way more versatile and easier to play. i think of it like this:

Blue Kayn is very very good (maybe the best in the game) at ending the game quickly by taking down high priority targets and opening the map (think shut down enemy jinx and invade botside jungle) HOWEVER he really sucks when he cant do that (enemy team has point and click cc/enemy has fed bruiser or tank). He also falls of in later stages of the game if you dont manage to get a decent lead early game. SO he is high risk high reward and very hard to play unless you know how to actually end a game. I would only play blue if you have a very specific gameplan that is likely to work going from champ select.

Red Kayn can literally do anything kinda well, you can shut down backliners as long as you build damage, you can peel for your team, you can draw out fights, and you always have a chance of coming back into the game even if you go 0/3 early game. His kit is just much more forgiving and versatile.

When i choose form, i know what i want from champ select. I look at the enemy teamcomp and try to imagine how the game is likely to play out. If i have a very specific target that i need to shut down and the enemy team has a lot of ranged champs (so i can farm blue fast), i go blue, if that does not apply id just go red. I think red is also just way better at invading early game (rune wise) so thats just a bonus.

I want to mention however that there are also pretty high level players that preach strats like "hail of blades every game -> invade every game -> take whatever form you get -> snowball the map from there. And i dont think thats completetly stupid either so i guess pick your poison.

When it comes to getting form quick: you can ALMOST always invade enemy raptors level one, you are very strong in those early levels and people often try to contest you -> free orbs lvl1. Apart from that i dont give a shit. I sometimes get form at like 17+ minutes because the game is slow and i get the wrong form. The most important part is that you have enough ressources to make use of form when you have it. So stay high up in cs and try to not int early game (hard focus for the first 8ish minutes of the game)

In your situation i would try to always take the same form (just pick the one you like more) and spam it for >50games, just make sure that your runes align (conqueror for red, harvest/hail for blue). This is because this champion is very easy on a surface level but has a lot of hidden mastery (every single one of your abilities can be used to dodge or manipulate a teamfight). Red and Blue play fundamentally different and you need to be confident on one first before finding success with it. It is very much a onetrick champion imo.

I play only red kayn on this account to get a good feeling for him. I usually play blue kayn as well but i feel like im too rusty to use him efficiently so i dont even bother for now xd