To summarize 1.29 conquest meta, it was
1) Move capital to Europe
2) Rush India -- only conquer non TC states/territories up until terr corr kicks in
3) Once you have enough econ, conquer non TCs
As a consequence of 2, you were not able to nicely chain your indian trade to your home node in the stereotypical collect in one place style, so you'd instead collect in a bunch of places and merchant placement/trade setup was therefore fairly trivial
In 1.30, the "conquest meta" (hard to say, since GC needs some more time to sink in) is
1) Get core good religion/culture states
2) Conquer territories + build courthouses on each province while being mindful of good trade chains
Aside from merchant republics that get it cheaper, TCs play a role in two ways:
a) getting a merchant
b) increasing goods produced in non TC provinces
The purpose of a is fairly obvious, and you would often target a when you know you need a merchant. b is a fairly underrated thing and is actually a very strong modifier later on to your "core states." For example in my only major 1.30 run so far as Ryukyu, I mixed states + territories in Bengal, Malacca, and Moluccas -- the goods produced bonus from a couple TCs massively increased my income.
Btw, to achieve a and b, you need your TCs to have high provincial trade power share. To do that you should look to lower the trade power on non TCs (for example delete market places, lower CoT level, and so on for territories/states) while increasing the trade power on TCs (CoT, buildings, etc.). Of course, do this only once you have almost 100% trade power in a node. One trick that I learned from an mp player was to TC low dev 5 province states and buy the 400 ducat harbor upgrade, which provides a massive trade power (more gold efficient than level 2 -> level 3 CoT) at a fairly low GC cost since you can generally find 5 province states with mostly 1/1/1 barren land.