I'll be honest, I have no idea what the Indomitus crusade is. I know it has something to do with Guiliman traveling around the galaxy, but what the actual goal, or what they managed to do I don't know. It only ever seems to reference a period of time after the fall of Cadia rather than an event itself.
Cadia blew up, Guilliman came back, the golden throne shut off for a moment, Khorne invaded Terra Guilliman saves the day by rallying the imperial forces, the Fabricator General tried making a deal with the drukahri to fix the throne(it's part eldar tech considering the webway project and all), the inquisition alerted the custodes to the fact that the drukhari wanted a trace of big E's DNA(after discovering the drukhari had made their own throne copy already) and put a stop to a part swap transaction, guilliman ousts half of the high lords of terra after they call in the minotaurs who fuck everything up, the fabricator general goes missing, guilliman uses the already sequestered imperial naval forces(they were at terra due to the khorne invasion) and starts the indomitus crusade to go see wtf is happening on the other side of the cicatrix maledictum because it's assumed that chaos is running rampant over there due to not being able to see the astronomican and no communication from there(it is a good assumption) and take it back by force if need be, the necrons start fucking about with their pariah nexus dead zone devices(cuts off the warp in large areas of space but also basically cuts people off from their soul turning them terminally lethargic, ultramarines aren't immune but can resist it for a time), Mortarion invades Ultramar so Guilliman heads over there to put a stop to it(dark imperium storyline), all while Guilliman's torchbearer fleets are spreading out to find chapters and give them Cawl's new equipment. Other Indomitus fleets are trying to find a path through the cicatrix maledictum, regain compliance of imperial worlds on the terra side, etc.
Meanwhile... Ephrael Stern is fighting her way through the cicatrix maledictum, presumably back to Terra. Kyganil fucked off back to the black library. Ahriman fucks up a 2nd rubric, reverses that part at least and then continues back to trying to invade the black library. Expeditions are sent back to Cadia for various reasons, mostly pilgrimages however Ashes of Cadia is a retarded book so don't bother reading it unless you want the most contrived bullshit girlboss story ever(yes, it's worse than the rest of everything else) from GW.
In addition to all of that, The Lion wakes up on the other side of the galaxy, finds out The Fallen are a thing and declares that to be fucking retarded and changes them to The Risen, puts the normal dark angels over there in check, meets Dante who wears a Sanguinius mask that pisses The Lion off but before killing him lets him speak and finds out Guilliman is running around again. So now The Lion is gathering up Dark Angels and Blood Angels, gets into a fight with Angron who previously smashed up some bullshit that caused an entire group of sisters of battle to turn to chaos with the rest of the fleet they were with(just chaos khorne style rage) while some greater daemon by the name of Vashtorr has convinced Abaddon to help him find a key(he wants to ascend to become a god, he's already kind of a minor demigod regarding tech or something) and the key turns out to be a copy of Caliban that he eventually fucks off with into space.
On top of this entire shit show. Cawl has sprung a Necron chronomancer from inquisitor jail(it was in stasis) and eventually goes back to the pharos device and tricks a c'tan shard into sending itself into a blackhole on the other side of the galaxy. Cawl in turn gets a map of all blackstone deposits within the galaxy, and uses it to trade favor with various forgeworlds(some like him some don't) because he promised Guilliman he could re-create the cadian pylons and stitch the cicatrix maledictum shut. While this is happening, Fabius Bile is hunting him and Cawl thinks it's because he has the Sangprimus Partum aka the instruction manual to create the primarchs(Guilliman gave it to him for the primaris project but forbid him from using the traitor legion info or the lost legion info). However Cawl had created Alpha Primus and Fabius bile wanted his gene-seed(and got it) noticing that Cawl had been fucking around and at a minimum made Alpha Primus using multiple primarch gene-seeds... including Magnus'.
GW retconned the date for the dark imperium story so it fit with the rest of the shit, and since then has not assigned solid dates to anything since. So basically all of this shit is happening either at the same time or within the first 10-20 years(maybe) of Cadia blowing up.
And then the tyranids invaded close-ish to Terra. I'm probably missing shit, but I'm trying to summarize.
The problem with not advancing the plot in 40k is that people get fucking bored. The problem with trying to do shit with the plot without moving forward in the timeline is that if you look at a pre 42nd millenium timeline 90% of the shit in 40k takes place in the span of like 10 years and it's idiotically dense. So instead we get moving the story along and vaguely moving the timeline forward because GW doesn't want to screw up having things take place simultaneously anymore or otherwise conflicting with other events happening at the same time.
