When I say I enjoy the ASOIAF conspiracy theories on youtube as much or more than the actual main story I am pretty serious. The world is rich and deep and fans starved of new content have wrote themselves years worth of filler and fun speculation. I definitely don't think you're undervaluing Fifteen Hours. The Horus Heresy and Siege is entirely heavy on Space Marine and Primarch perspectives. The writers are aiming for greek myth down to demi gods and culture heros mixed with historical figures. As we have gotten closer to the end there has been a lot more inclusion of the baseline human or Mortal perspectives. There is a limited throughline from the very beginning of a few very important mortal figures that have carried into the conclusion of the heresy, and those storylines reaching maturation has been overall pretty cool. Specifically, certain aspects of the Imperium of 40k culturally are directly determined by how the Heresy found its final conclusion. I can point to an immediate small example that on Terra in 'contemporary' 40k specifically there is a population level terror of Adeptus Astartes that was not present in 30k. That bias exists because an entire Legion of traitor Astartes spent a few weeks-a few months turning the entire population of a medium sized country into snortable, injectable, boofable drugs. The process being excruciating and extended for the victims. So a lot of what I find enjoyable about 30k is it gives the historical explanations of many different minutia of the Imperium's culture and functioning. The second example I would point to would be the formation of the Black Templars, specifically with the ascension of the first Emperor's Champion, Sigismund. As someone who has love for Dorn and his sons, you would appreciate the absolute LOVE shown to the Fists during the entirety of the Siege. I don't want to spoil too much, but in my mind the tension between the Fists focus on the material world and only taking into account the hard truths of things and the open acts of divine intervention that have protected their leaders and line brothers alike is SO. COOL. The Siege is worth reading alone even if you don't feel like slogging through the entire giant heresy.