The only other alternative since the hidden noble rebel nerf is revolutionary rebels. You have some idea-group chance for them in both aristocratic and religious.
As for good colonization ideas, trade is reasonable as you're conquering into an end node and it will give you a bit more naval FL, more merchants for steering, and more money from it. As such, it's one of a few respectable early picks in Ireland. Unfortunately, it's hard to justify economic in a similar fashion.
Influence and Aristocratic are reasonably good idea groups in general for vassal feeding strategies, as you get more diprep, more FL from vassals, and aristocratic hands you an extra diplomat, but there's not much money here and it'll delay colonization.
Basically, IMO you should just go trade --> economic --> colonies (work in a military idea there somewhere too, maybe quantity to spam buildings with economic) or pick aristocratic or religious and hope for the best. There's no really good method AFAIK.
As for the colonial game, you can definitely leverage trade + ships to trash competing fleets and take colonies for yourself, then grab a colonial idea later and keep going, but you could even realistically have a heavy/dominant colonial presence despite being incapable of making colonies yourself.
