I am french so I will use google translation to make me understand :
In general your skript is pretty good but there are some essential points that spoil everything :
1 : You should no longer use SkQuery or Skutilities to create .yml files. It has been proven that it ENORMOUSLY lag your server (
https://forums.skunity.com/threads/dont-use-yaml.3850/).
There is an alternative is the Skript-yaml addon, you can find it here (
https://forums.skunity.com/threads/skript-yaml.5798/).
2 : Your script is very poorly optimized. For example, where you give kit items to players, phrases that remove potion effects should be placed at the beginning for all kits and not in each kit. This is an example of so much other optimization problem.
3 : Finally, there is a big problem on the command, the players will be able to use kits command many times. This should not be possible.
Here are some of the problems I saw at a glance.
I can not help you correct everything in comment, if you want help to solve the problems you can contact me in private message. I made myself a skywars in skript not long ago (not shared here), so I have in mind all the mechanisms of this game.
I will see this topic later, to increase my rating if you correct the problems :) Good luck.