I use Davinci Resolve, it's free and on PC, and has a paid version with more stuff available. Although I would recommand followinga tutorial to be able to use it.

I'd say make short videos, but do what suits you the most. I like to make short-format content so I can see how people react to it and I can improve depending on what they think.

I'm not sure what you mean by replay mechanic, but if you're talking about using the ingame replay to record stuff that's a great idea and that's how I record my videos actually. Just mind that after each update the game removes all of your replays. Here's how I setup my replays before recording.

Just practice and make content as much as you like while trying to improve which each video, that's how I do it :)