On the issue of conquering the Ferengi: if the pirates did that, they'd just get a bit of wealth and then nothing more. The Ferengi aren't rich because Ferenginar is rich, they are rich because they constantly work in gaining more wealth. If they were conquered, they'd stop doing that and poof would go the wealth.
Much better to accept some Danegeld from the Ferengi, because that source of income will never dry up as long as the Ferengi are allowed to stay free - or as free as their "mercenary arrangements" or protection racket allows.
I sort of like the novel/RPG/fandom idea that the Orions are a very old empire in decay, or indeed well past decay. Such an empire wouldn't necessarily be limited to just one species, explaining the diversity of Orion portrayal so far. And many different factions would claim descendancy from the original empire, there being some claims that might hold actual validity, and many more from opportunists who were not really related to the old empire at all.
By the 24th century, the Syndicate may have become the dominant expression of Orion might, even if it is by now largely unrelated to the Shrek-skinned, green-skinned or blue-skinned Orion species/races of ENT, TOS and TAS fame.