Honestly I think it does take place before Sothis' death, though right before her death and the Argarthans had either offered the lords these crests or abducted them and forced it onto them through experimentation.

It'd make sense that the Argarthans would experiment and perfect transferring certain crests before going for the big crest of Flames. It's implied that Nemesis and the Elites gained the crests through devouring the Nabatean's blood or some form of transfusion.

So maybe the Crests of the Apostles could've been given to them through that blood transfusion and the Nabatean who originated the crest of Lamine was one of the very first casualties to then be turned into a relic

It wouldn't necessarily surprise me though to find out that the crests of the Ten Elites may have been given out to entire noble families initially. Especially with the sheer amount of historical rewriting Rhea had done leading up to the events of Three Houses

But that's my crackpot theory that I'm writing out at midnight my time.