The colonization techs are time intensive to research, only useful in rare and specific situations
So true - they're very expensive and usually you only have one or two planets (until you start conquering) for which they're useful.
and the AI wont trade them (generally speaking)
It depends what you offer. The colonization phase of my games usually has two smoothly-interlocking halves, joined together by a single session of tech trading.
Part 1 involves scouting the nearby systems and grabbing everything that isn't nailed down, even PQ 1 and 2 planets (they're SO useful later, after you've terraformed them to PQ 15-16 planets, and I like to get their population started asap).
On the turn where I've run out of habitable planets to colonize and my next colony ship can reach the best 'other' type of planet I plan to grab, I take a quick look at all of the other races' techs before deciding which one I want to trade with first. Once I've picked out that first sucker - the one with the most planet techs that I need, and the most other techs in a tie situation, I offer that faction a research treaty. Normally, unless my diplo rating is abysmal, they will cheerfully give me every tech in their posession, their space mining ship, and a few hundred bcs in the bargain. If I'm very lucky, sometimes I can get them to throw in a reciprocal research treaty (?!). Then I take all of the techs I got from that race and trade or sell them to all other races because hey, they're going to sell them anyway, so I might as well get the cash instead.
Then I usually have no trouble gobbling up whatever other planets I need or want before I switch over to industrial and military buildup.