“Viable” depends solely on your ranking. If you’re playing at highest level this might be true but for those who play at lower levels it not just only viable way to play.
Also remember 2TC + Song = 3 TC’s. Chinese boom is not that special when compared to some other civs that get cost reduction on vills, economic techs or something else.
Also when going to 2TC + Song, you spent 450 food on every minute to make villagers and each villager gathers around 40 resources per minute and villager costs 50 food it takes over 1 minute to villager turn into profit. This is very important, because more TC’s you make, less resources you will have at your disposal for short period of time which translates less tech, less army and so on. This is one of the reasons why Abbasid boom is one of the best in game and why Chinese boom is not special it becomes better the more you build TC’s but its not really advisable to go more than 2TC + Song.
Song dynasty gets vastly better the more TC’s you build so essentially your only advantage over someone who goes 3 TC’s is 150 resources and little bit extra (100 resource difference between TC and Song + Mining camp + villager travel time)
What you should try is to play around the china and see what works for you. 1TC + Song and later 2nd TC in castle, 2TC + Song or 2 TC and later Song. Try find your own style.
When it comes down to defending early boom its important to understand layout of map. Some maps its easier to wall of than others but tactical walls to limit the movement of opponent is also big factor. When your match starts up and you start scouting around etc, try to think how you can wall yourself in most effectively and how to limit opponents movement.
Another thing is (I start always with 2 scouts and because I play 2v2 there is more sheep) I place my initial mill under TC while leaving 1 tile between TC and Mill. This way I have safe food source under TC which is completely protected from everything and insta delivery for taxes. Usually my mill is between gold and TC so so this way I can place imperial academy always on 2 most important resources and have insta delivery for one of them also to be frank its easier to get 3 resources or even 4 this way under influence of IA.
Also almost always use barbican defensively. Its my style. It protects my gold, food and most likely woodline too. I don’t see much of point dropping BBQ on middle of map and wasting all the time for villagers to travel there. Sometimes I do drop BBQ on middle like on danube river where I can cover up 2 boars and berry bushes this way I got lot of food at my disposal, but again I try to reach castle as fast as possible with prof scout and song dynasty.
Also consider which landmark to go with first. For example against booming type off play always go with IA, but more aggressive civs it can be better to make BBQ first, but its very situational.