So, am I the only one who just doesn't... enjoy republics? It may have something to do with the fact that in the Elder Kings, the Heartlands republic begins as head of state, meaning virtually nothing in terms of levies, which tends to result it its being gobbled up in less than a decade.
It seems to my though, that when I play a republic in general, times just flies and very little happens to engage me. The whole dynasty business simply falls into the background, and military conquest never seems viable, so you just bide your time, waiting for the day you are wealthy enough to mercenary roll the continent.
Getting wiped out quick is more of a problem in Elder Kings because of how their CBs are. Overall I think republics are fun enough since you still have access to all the other mechanics, but they do take some finangling and you have to be fine with spending a lot of time relatively inactive. Works for me since I can have CK2 on one monitor and chrome or something else on another, which is how I play most games anyway.
How do you get the trade republic as a pagan?
Play as a tribe with a coastal capital, and then do it the same as reforming to feudal, but click the "republic" button instead.
Extra broken with Norse, since the raids let you get both money and prestige for re-election and other purposes.
You can do the same thing with certain cultures too, like Berber, which is what my Republic was.
You don't get all the other mechanics, seeing as how you can't change succession laws, nor can you change to an Imperial government type thing, which means you'll be 'aving more than a few king vassals once you hit that limit.