Is there some cultural model for ancient Suel empire? I mean, ancient-Roman-like, Byzantine-Like, Francia-like, and such.
There's very little official material to go on.
We know what they looked like (tall, pale, blond) and that those that survived the Rain of Colorless Fire lived in migratory tribes.
As you've already mentioned, the
Scarlet Brotherhood base their society on their idea of what ancient Suloise culture was like, but it's presumably grossly distorted by their racist extremism and the scant records from that era.
There's the Gods of the Suel articles from some early Dragon magazines but those didn't go in much for cultural info. I guess you can at least tell what the Suel considered to be important from what they had gods for. Most of them are fairly standard - Time, Strength, Magic & Death and so forth - but it's noticeable they had gods for Assassins, Treachery and Bigotry. One gets the impression some Suel weren't terribly nice people.
The only named ancient Suel I can remember off the top of my head is Izek the lich from a Dragon magazine side-quest.
Gary Gygax's wrote a novel
Sea of Death in which Gord the Rogue novel entered a buried Suel city under the Ashen Desert, one that was inhabited by Derro (who are descended from Suel who got trapped underground according to Greyhawk lore). It's been a while since I read it, but I remember there was a big domed temple hall. The book cover shows it having Greco-Roman style pillars, although I don't recall if that was accurate to the text.
At least that tells us the ancient Suel had sophisticated stone architecture, so I guess their building technology would be roughly on par with ancient Rome.