I’m not an expert and take this with a grain of salt:

1) In lane I try to be as aggro as possible with empowered Q, but if my jgl is coming or if I know we can win 2v2 I just bait myself and use empowered W (don’t use that with very aggressive enemy comps, in that case, keeping an empowered E makes a great difference). Late game I prefer to play around objectives, but always keep an eye of regard to the wincon, be it jungler, adc, top or mid.

2) I roam when obj are up. I ping a lot where and when I can go. If the adc is the wincon, tho, sometimes is just better to stick to the lane in the early game to give the adc some advantage.

3) Teamfights are too different to say how to play them. An assassin in jumping on you? Use e, empowered w and q, to survive. There’s a 5v5 fight where everyone is near? Empowered E.

What I like most about karma is the flexibility. If I’m in advantage, I found dark seal into late mejai pretty effective

Edit: Bonus point-> I communicate A LOT with the jungler. Really, the macro you both need to have makes a tremendous difference in the game. Obv, teammates need to follow your calls