Late reply, but here's the deal:
Septarians are immortal. Immortality means low birth-rate. If anything, Septarsis isn't a "nation" as much as it is a "force", a group of migratory lizards who come and go as they please. On any map that we've been given, Septarsis hasn't been shown at all, so either it's to the far east of the Avarius Kingdom, or it's simply a group instead of a land. This, of course, puts them in direct opposition towards established kingdoms like Mewni, because these empires take up clay that could be used for their foraging. The Great Monster Massacre means little to them because they couldn't die; what did matter is that they gradually lost places they could migrate through, because even though they can't be stopped, they can be repelled.
During the Monster War during Moon's time, the little monster with the crown that we see is less of a leader, and more of a figurehead. His authority isn't over the real, dangerous monsters, since him and his followers are eager to seek peace. Toffee, on the other hand, is a real, dangerous leader, and was someone the real threatening monsters followed. Whether he is royalty or not isn't relevant, anymore, though a part of me believes that they included the title "prince" just to hurt Moonfee shippers.
There was something in the first guidebook that referred to the Avarius-Septarian Treaty, which means at some point these two nations collided. What makes the most sense is that, following Toffee's defeat, the monsters were appointed a leader by Mewmans, and those leaders would be the Avarius'. Given how Moon treats the Avarius family in comparison to other monsters, it would make sense that Toffee and his remaining forces attempted to usurp the Avarius', and ultimately failed for one reason or another. Toffee went into hiding, Rasticore was shuffled off to keep an eye on another Butterfly, Meteora, and one day after this, Ludo was born, ignored, abused, and unaware of his own family history.