I have rewatched the Harsiesis related storyline. I believe something is still missing from the timeline. What could happen with the Goa'uld why they don't want to remember their past crimes? They are evil anyway, so what can be even worse than them? There must have happened some serious events, but unfortunately the SG shows have never really explored this area of their past. I have imagined a little quick backstory. We know that they were taken the Unas as hosts. Then maybe some other visitor race was also taken as host so they could leave their planet through the stargate or some spaceship. Maybe The Great Alliance has seen a great potential in them as they wanted the "good" Goa'ulds to look after and guide the human worlds while they were escaping to the Pegasus galaxy. That is the reason they were named as System Lords, as literally one Goa'uld was guarding one solar system. Maybe the Great Alliance has already fallen apart that time and this step looked like a good idea as they Goa'uld hosts looked immune to the brain infection, but no Ancients was willing to unite with them, I don't know. They were spreading and they could controll easily most of the galaxy. There were complete worlds where everyone were Goa'uld! Then they have meet an alien race on a distant world who wanted to stop them and they rebelled against the Goa'uld rule. The free Titans have used advenced technologies (like laser firing satellites), but they couldn't defeat the complete Goa'uld as there were so many of them and they infected so many races in the galaxy. One of the minor Goa'uld Queen (Mnemosyne - goddess of memory and in this case literally the genetic memory) made a biggest evolution jump for the Goa'uld. She has taken a Titan as a host. She learned that host and the symbiote can share memories and experiences, so these essential information can be coded into genetic memories. They learned all the technological defences of the Titans this way, so more and more Goa'uld wanted a Titan host as they were jealous to each other. The Titans were really effective enemies in the past and most of the Goa'uld worlds have been wiped out easily. What Mnemosyne and the other Goa'ulds couldn't know that the Titans decided to have an evil long term plan how to defeat the Goa'uld society from inside. They artificially engineered Titan bodies which directly sacrificied themselves as hosts, so the first Titan was misleading Mnemosyne (her host name was Gaia). We know that the Goa'uld Tel'chak (Mayan rain god from the season 7 Evolution I-II) found that cube like device in 900BC which helped to create the first Goa'uld sarcophagus what ruined the souls of the Goa'ulds. The Titans directly wanted the Goa'uld to find it, so while the Goa'uld could heal themselves, now they could heal their vulnerable aliens hosts as well with the cube. Later the developed sarcophagus technology what looked very-very adventageous in a galaxy wide war. The proud Goa'uld haven't realized the side effect of the technology in time. They became more and more depent on the Titan technology as every Goa'uld wanted to live forever. They became more and more suspicious and hostile to each other, while the genetic memories made sure that all the future generations of the Goa'uld were the same as well. They almost killed each other completly as they were fighting for more and more power. Not to mention that they symbiontes became parazites. Unfortunately the other side effect of the technology was that their life can be only extended if they were consuming other Goa'ulds, so that is how the cannibalism started. Meanwhile the Goa'uld Ra discovered the Tauri, where they found a new, easier controllable host race : the humans. Most leaders of the Goa'ulds all changed to human hosts as this race looked perfect to repopulate the galaxy under their own terms and they killed most of the earlier host races. (The legends would come from here as the first generation of gods in Titans bodies, then the rebellion of the Titans etc.) The Goa'uld made an agreement that the Queens should not pass the Titan DNA to the future symbiote generations. They couldn't control the reproductive system of their own human hosts, but at least they could completly control their bodies not like the earlier host races. So they also discussed they will not allow of two Goa'uld humans hosts to have a common child (the Harsiesis) as even the human hosts were carrying some part of technological memories in the Titans DNA (in the quiet part of the human DNA) what was more stronger if it was doubled in the DNA of the common child. Some Goa'uld tried to use the Harsiesis as hosts, but they couldn't complete control on such hosts and they became completly mad (as a genetic revenge of the Titans), so most of them have rather abadonned and killed the forbidden childrens. Even these became such myths what they never ever talked about again. So eventually the Titans were defeated and a few survivors were imprisoned, but their mission was basically successfull. The mind (and the genetic) of the Goa'uld was infected forever and even the Queen couldn't stop of spreading it. They never tried to take a Titan as a host again. An advanced and safe human host was always a long term plan of the Goa'uld. They really made them dependent on the sarcophagus technology and their numbers greatly decreased because of the civil war and the cannibalism. What the Titans haven't calculated in that millions of humans suffered from slavery and Goa'uld civil wars because of their genetic memories and revenge.
I know it is not the most exciting mini-story, it is just a quick improvization. We know that even Ra has had a strange alien host, maybe that was the reason he could gain power as he was the one of who found Earth and the humans. Mnemosyne could be left out completly and he could be the one who has taken a Titan as a host, but Ra has never been a Goa'uld Queen as he had Hathor, so it would look a bit odd how his actions had an effect on every other Goa'uld around him. I know I don't have to expalin everything all the time, just I don't like plotholes.
And we should explain what happened with the earlier humans in the galaxy? Were they all Ancients? Or have all of them died out / ascended / escaped? What if Ra's host was a Vanir (see the small flashback at his death in the Stargate movie)? What if they wanted to ruin the Alliance with the viral infection of the Ancients? That would be also a big revelation that they have been manipulating even the biggest races for a while now.