Don’t worry about On or Kun. WK will teach you the reading you need with the lesson item. Think of it as just different ways to read something. For example, 力 by itself as vocab (purple background) is ちから, but as part of a compound Kanji like 入力 it’s read as りょく.
Learning some grammar will help here. For example, 大した is set phrase and the した part comes from the verb する.
Definitely. If this is the first time you’ve tried to memorize things on a daily basis then it will be hard at first, but you’ll eventually get better at it the more you do it. It’s just like any skill.
Just keep doing what you’re doing. I’m willing to bet you will get better at it after a month of reviews.
That’s ok. We all feel this way to some degree in the beginning. Don’t let it stop you from continuing.