Happygeek, I'm in a similar situation to you in many ways (haven't played multiplayer, really like the single player experience, worried how much fun MP would be), and I've also spent a lot of time reading around the forums to figure out if Multiplayer would be fun for me. If it'll be any help to you, here's what I've personally concluded:
While there are certain tactics that see lots of use in late-game multiplayer, I feel that they aren't as specified as you may think. Rather than late-game players focusing on Tarts, for example, think of it more as late-game players employing SuperCombatants. It's not that Tartarians are required to be successful in the late game. Rather, the amount of magic and strategies in the late game grow to the point that players need to have SuperCombatants to act as all-purpose offensive and defensive units, when it's too difficult to build an army of units, commanders, and mages to account for everything that could be thrown at you. It's much easier to make one unit that can deal with most things pretty well, because making an entire army that can do so would be very wasteful.
Now, the simple fact of the matter is that Tartarians are pretty darn good bases on which to build SuperCombatants, but they're far from the only option. Golems are pretty common SC choices, as are the various Angels and Demons. I wouldn't think of it so much as "I *must* use Tartarians to be competitive." Rather, I'd think of it as "In the late game it's very easy for people to make big, strong units that can destroy whole armies single-handed if I'm not prepared for them." It's a rock-paper-scissors sort of thing, and the foundation of all strategy games. There are strategies and counter-strategies to them. The weakness of SCs is that no matter how well you prepare them, they'll always have *some* shortcoming, some facet that isn't perfect, and you can make anti-SuperCombatant thugs to capitalize on those deficiencies. And then your opponent will figure out ways to beat those thugs, then you'll figure out ways to beat that, and so on and so on. It's the same for every strategy in Dominions, there are who-knows how many strategies you can pursue, and then there are who-knows how many strategies to counter those strategies.
But anyways, back to your (and my) concern that every late-game period in Dominions 3 might be the same. Have you read through Baalz's guides?
Here's a link to a topic with all his guides if you haven't seen them. They're pretty enlightening in general, and he addresses a lot of the common late-game tactics as they apply to whatever nation he's writing about and will introduce nation-specific strategies that break the norm for you to emulate and build on. For example, I can't think of a single Baalz guide that doesn't detail one of the nation's specific units that makes a great, and often *superior*, option for a Super-Combatant to the typical "build-lots-of-tarts" tactic. His Bandar Log and MA Ulm guides come to mind, where he describes how Ruddras and Iron Angels make great SCs, or his EA Caelum guide that shows how Eagle Kings can be used from turn 2 to the last turn of the game to counter and defeat anything your enemy can come up with.
So, again, try not to think of things in narrow terms. That's what I did at first too and it really discouraged me from giving MP a try. I still haven't gotten into a multiplayer game, but now that I'm thinking about Dominions 3 in a "there's infinite ideas, and I'm in this to have fun" rather than "I-must-use-these-common-tactics-to-win" mindset I really can't wait to give it a try
And I wouldn't worry about the other players being rude, I just can't see people who play this game having that sort of mind-set. Could be wrong, but at the very least everyone on these forums seems pretty nice <_<