Have you read any of those Dark Imperium books post-Guilliman's awakening?

Yup. I wanted to enjoy them, and certainly enjoyed parts. The more perspective we get from guys like Frater Mathieu and other mortals the better. But overall I felt they were pretty bland. Ultrasmurfs versus Plague Bois is only entertaining for so long. By contrast I thought Spears of the Emperor was AMAZING and is what more warhammer novels should strive towards, specifically because of its emphasis on the relationship between transhumans and mortals.

HH grew lukewarm for me, but those new developments turned my head a bit. I don't care about spoilers, but only know the broad strokes so far.

The Horus Heresy for me is best enjoyed as a series of moments, vignettes. I LOVE the Siege of Terra series so far specifically because it takes the broad strokes that were fated to happen and gives context to the big events. I will agree though that there's lots to slog through before you get to the juicy bits.

Overall I'm pretty happy with the direction GW is taking with the franchise, mostly because they seem to be a lot more free with the IP and there's lots of other good content out there. Hammer and Bolter the TV show has been a pretty big smash hit amongst the fandom and I'm loving the Warhammer 40k: Darktide game. It's a bit barebones but the developers are committed to supporting it for a while, and its core gameplay loop is fun for me.

Cybulskie's book sounds interesting. I like that kind of side-view take on history. I may add it to my list once I'm done with War and Peace and Buddenbrooks.