For chaos The Ahriman omnibus follows Ahriman from thr thousand sons legion after the heresy and is a fun story. It's got lots of thousand sons warp fuckery and some great characters.
The night lords omnibus is one of the first 40k books I read going into 40k with not a whole lot of knowledge. It's still probably my favorite 40k book, the characters are written incredibly well, the dark moments of horrific comedy are there, it features some great fights.
Fabius bile omnibus follows fabius bile of the emperors children legion in what's essential a great mad scientist story. Once again great characters and a fun story definitely worth the read.
The lion son of the forest is a very new 40k that follows the lions return to the setting. From what I remember it doesn't require a whole lot of backstory to be neccesary, as it explains a lot along the way. It's very involved with some dark angels space marine characters so I could be a decent jumping off point of sorts.
I usually watch YouTube on a laptop hooked up to my tv, so I downloaded freetube and just imported my subscriptions. It's got sponsorblock and other features built in, but you can't like or comment on videos. Overall though I've been using it for about 6 months now and wouldn't dream of going back to YouTube and trying to constantly deal with them fucking it up more daily.