I gather a big issue here is our lack of a storyline exclusively dedicated to the Romulans and their intriguing past. Like Flintheart Glomgold to ol' Scrooge in Carl Barks' stories, the Romulans were extremely generic throwaway villains for most Trek writers, changing characteristics (and appearance!) from story to story - but mistaken afterwards for well-defined arch-enemies. If totalitarian worked better than meritocratian-aristocratian, then totalitarian it was. Klingons were originally the same, but then TNG happened...
Klingon history can now be parsed together from Klingon-centric stories, which benefit from Klingon principal characters and guest stars. Romulan history is basically "Gambit II" pasted onto a background of fan assumptions and early novels, though. And as MAGolding points out, while the writer intent seems to have been to do that very pasting, everybody having read their Diane Duane and all, it went sorta bass-ackward, with "Gambit II" containing some dubious timeline references and the rest then trying to patch that up without acknowledging the "Gambit II" choices in full.
How dubious is dubious, though? The Romulan story goes back whole millennia, rather than the centuries involved in Klingon or Cardassian history - so a reference to 2,000 years may mean anything from 1,500 to 2,499 years for the careful speaker (Data or Spock), and anything from, say, 1,300 to 2,700 years for the average speaker.
An interstellar empire collapsing into a single planet may take all of two days. A single planet spawning an interstellar empire may happen quickly as well, though, as we have seen how trivially easy it is to build warpships once the secret is uncovered. If S'Task had a well-equipped garage, he might have built a fleet of ships in a year and founded Romulus over a weekend, simply by pitching some tents over suitable wetlands on a hospitable Class M planet.
The rest is interpretation of disjointed tidbits. Romulans and Vulcan colonization go together - but that doesn't mean Romulans were a Vulcan colony. Rather, Romulans might have been the first to carry the old Vulcan conquistador spirit to the stars, fighting brutally among themselves and everybody else just like the ruthless colonizers of Vulcan's Slightly Moist Mountains and Great Almost Livable Canyon had once done...
Timo Saloniemi