Maybe Tang ends because of economic reasons, land accumulation and loosely control of marginal area. For example, Tang defeated Japan in Korean war I, but it didn't put political suppress or economical punishment to Japan. In this case, China didn't gain any benefits from the Korean war and Japan didn't pay its real cost of invading Korea except its army and navy death.
I hold opposite opinion of yours, Tang started too few external wars, not too many, this strategy diminished its influences over the whole area, compared to Han dynasty.
When a country has absolutely power to punch its counterpart, war won't bring serious results.
Another prior aspect I analyse, most Chinese dynasty indeed sacrifice super power's rights when living together with neighboring nations. For example, China historically help Vietnam to solve their dozen time internal farmer revolution, but China pay the war ourselves. It's Vietnamese affair, but Chinese pay for weapon, food, transportation, lives, everything. And Tributary Trade System with most Asian nations. It's a huge burden to China. It likes we use silver to exchange earth, it's not trade, but China synchronously didn't require plus political or geopolitical benefits.
You need to read more about Tang history. Put all events in the right calendar.
In 745, Tang crushed Later Turk (后突厥) and completely ended Turk political legacy in Chinese lands. Basically at that time, in whole China, including Siberia, there were no forces not submitted to Tang. Then Tang started aggressive adventure in Central Asia. In 749, Tang conquered the whole Afganistan (石国). To stop the advancement of Tang, the new Arabic Empire leaded alliance of many local tribes and had the Battle of Talas with Tang and its allied Tribes. One of Tang's alliance rebelled and Tang lost 20,000 soldiers in this War and had to retreat from Afganistan.
If Tang didn't lose Battle of Talas (恒罗斯之战)in 751, An lushan (安禄山) might not dare to revolt in 755.
Tang had the thought to control the whole silk road. Chengis Khan just followed Tang's route and went further. Just similar to current United States wanting to control all trades. Empires without a major loss always think that they can achieve anything in their mind.