Played my own Massalia game, managed to take out Rome in under 40 years.
Granted Carthage did some of the heavy lifting in a war that they concluded right before I attacked. Romans still greatly outnumbered me though and had like 3x the pops.
As far as senate support my takeaway is:
Oligarchs: Great (They get happier with tyranny and grateful families which makes them easy to keep at max happiness)
Traditionalists: Also pretty good (Gets happy when you have all deities from your state religion). Downside is they hate low stability, so changing a bunch of gods around early on or getting lots of AE will hurt you.
Democrats:: Fucking awful (get massively unhappy with tyranny and grateful families)
You can find these modifiers by the tooltips here:
Unfortunately the % control of each faction is very hard to manage early on because the heads of each family have most of the power in your nation and you can't really affect which party they choose to be part of. Once you've expanded you can start granting offices and provinces only to traditionalists and oligarchs and get loyalty high easily. Early on I had like 70% traditionalist influence just because all family heads were traditionalist but now a bunch of the fuckers are democrats.
Notably the faction of your ruler will give +.2 to your party and -0.14 to other parties. Since normally your leader is of the strongest faction (they have the most powerful members after all), this tends to be good. The top left character in your offices screen will specifically give their party another loyalty boost, so I recommend making that character the strongest oratory member of the Oligarch/Traditionalist party, whichever your ruler isn't (hopefully you never end up with a democrat ruler, I think you're just kind of fucked then).