It depends on what you want to get out of it. Are you looking to start a YouTube channel? If so, what are your expectations? Retro PC gaming is already pretty niche and YouTube is already saturated with "Let's Play" channels. It's unlikely that a new retro PC gaming channel will make it big today, especially if it doesn't offer anything new and unique. However, if it's something that you enjoy doing and don't mind having a small audience, why not?

I never record myself playing games, but I could see myself recording something in the future if there are no gameplays videos on YouTube. I find it helpful to check out gameplay videos before buying games myself. Otherwise, I don't really see the benefit.

yes

I don't twitch but I have recorded a 1 credit clear of Night Striker recently (only posted on a couple discords, was a fun encoding experience to get 15 mins to 25mb).