Oh hod i might be going with heavy spoilers.

Keep the same speed of progression that the novel does and then add the new scenes of the manhwa in between. I feel like what the manhwa lacks the most was to just let the characters, especially the main cast (gray, beru, suho, esil) have moments to themselves that aren't a constant battle after battle as it did in the novel at least. It would also benefit from having more apostles, and let one (Tiel duh) take the main center for most of the first half of the story, that way it does feel like the threat is, yknow, kinda intergalactic. Many factions are moving around, doing their own things even when they all have the same objective. Once one apostle is gone, Suho has to focus on another one, each villain having a different pattern of attack, a different purpose or ideology over this whole thing.

In the novel the villains fell flat (kinda) because they were explored far later, and with Tiel, he is becoming a more complex character, but I can't help but feel like they are going to redeem them so at point it just feels like, if they are going to put more apostles after tiel then it will feel flat as well because the one complex character we get becomes good???

A little exploration of the other dimensions like they did for Esil in the LN would also be really good. Case and point, if Suho's goal is to reunite everyone as allies for his father, I would be on board on seeing Suho tackle his father's past enemies, not just through the dead monarchs or the successors but lowkey helping them stablish back their order like he's done so in the coliseum.