So now I know what I like, but I still don't know how to identify it. What are the general categories of types of tea that I should look for to explore more of this last kind of oolong?
Oolong is a bit of a misnomer, as it is only one kind of semi-fermented tea under many others.
But i can understand you completely. What you drank and did not like was one of the more flowery less fermented and less fired semi-fermented tea, such as Oolong, modern Ti Kuan Yin, or one of the Taiwanese semi fermented high mountain teas. There are many different qualities, naturally, and some of them, especially from China, may be somewhat unnaturally enhanced in their flowery taste.
I only like those greener semi-fermented teas on occasion, and in between.
I am also not a great fan of Dancongs.
I do much prefer the more earthy Wu Yi teas, such as the Bai Ji Guan you liked. High quality Wu Yi teas are my first love actually, and i do prefer them over Pu Erh teas. But they can be extremely costly and are very difficult to get.
As to brewing: these tightly rolled greener semi-fermented teas need less tea in the pot. You cover the bottom of the pot, at most one third of the pot, as the leaves expand massively.
For Wu Yi teas there are different ways. The cheaper teas of lesser quality often need more leaves. In the Gong Fu method you do at times almost fill the whole pot with leaves. I found though that the very rare top quality Wu Yi teas need only one third to half of the pot filled for the best result as more tea leaves can suppress the finer nuances of those rare teas.
Wu Yi teas especially can be aged as well with excellent results, but very different than Pu Erh. The first few years Wu Yi teas should be aged to lose the taste from the firing. But later then taste increases in depth. I love, for example, the changes in taste of aged Shui Xien. This though works only well with high quality Wu Yi teas. Only good leaves will age well. Low quality teas will stay low quality.