Which quests can I skip to jump straight into newer content?

Hey folks, I’m a returning player and starting fresh on a different console, so I have no existing character. I’ve played on and off since launch, so I’ve already done the early main quests/Wastelanders stuff multiple times over various characters.

I stopped playing after Wastelanders, so I'm mostly interested in the stuff that came out after that (Steel Dawn/Steel onwards). I know the game lets you approach things more freely now, but I’m not sure which quests are required to access the newer content.

Specifically:

• How do I skip the majority of the original main questline and just go straight to joining the Enclave and nukes? • Are there any requirements before starting Steel Dawn, Skyline Valley, etc? • Will I be underpowered by skipping quests and missing out on rewards?

Basically I just want to fast track to the new stuff, and leave the older stuff until after, if at all.