I am not familiar with the specific military innovations made during the Qing in the 18th century. So I will refrain from answering that part of the question. I apologize ahead of time for typos.

In terms of affecting foreign policy, it did have a very dramatic effect, at least over time. These were generally costly conflicts that, generally speaking, brought little material gain in the short-term to the empire. Xinjiang and Tibet, two major targets of Qing military efforts, were not economically as significant then as they are now. Today, China derives heavy metals, control of hydroelectric power, fossil fuels, and some impressive agricultural production from these regions. However, none of these would have been economically or technologically possible during the 18th century. The campaigns led subsequent emperors to curtail direct power projection abroad and seek to re-balance traditional tributary state relations.

To make things worse, these campaigns were ridiculously costly. It's hard to pin down the population sizes of these regions before the campaigns, but likely only in the thousands. The one exception was Vietnam. Vietnam was sizable then in terms of population, but it was never fully incorporated and brought no additional material benefit compared to the previous status quo. The cost the campaigns has been estimated at 151 million taels (a weighted piece of silver). It's hard to gauge the total effective tax revenue that the empire brought in, but it was likely in the tens of millions of taels per year. At least one estimate for the early 19th century puts this figure at 45 million taels annually. In short, 151 million taels is a lot, and it was major waste of financial resources to pursue these campaigns.

But were the campaigns the cause of the empires decline? Well, not really. They were a myriad of causes that led to the decline of the Qing that go beyond simply poor financial management. On the technological side of things, the most obvious field where the Qing were deficient was in maritime technology. Chinese ships weren't technologically advanced and the Qing Empire, much like the Ming before it, saw investment in the state's naval capabilities as secondarily important. This decision is logically to an extent, China was the largest economy for most of human history and could produce domestically virtually every good and service. Why invest in getting access to new goods or establishing trade relations if there is not any perceivable benefit? Unfortunately, during the last 500 years of human history, it has been naval power, not land power, that has brought about the rise of global empires. Financially, the benefit of better access to and control over global trade flows is obvious. From a military technology perspective, it gives states increased access to new ideas and weaponry developed all over the world and better access to the resources needed to produce these new weapons.

That's one factor. The second factor that led to military decline is less obvious, but ironically was a byproduct of the Qianlong's enormous success in domestic management. The population of China in 1750 stood at 181,810,000, by 1800 it had grown to 332,181,400. This is huge by contemporary standards, but it was absolutely monstrous by 18th/19th century standards. This population growth came about because of a relatively stable domestic political situation, the introduction of new crops from the Americas, and improvements in agricultural techniques and technologies. While more manpower is good, it also means that there are more people to police and control. Moreover, it started a trend that would persist into the 19th century of lower caloric yields per capita. All Chinese farmers could do oftentimes to get more food was to throw more labor at the problem.

Overtime, agricultural production fell behind, creating an even greater need to devote resources to maintaining public order. Much like the Ming Empire, by 19th century the Qing's army had largely transformed into a glorified police force. If the goal is simply to maintain public order (instead of competing with rival powers), then the development of military technologies become less critical for success.

In short, these factors led to the decline of the fighting capabilities of the Qing Empire and its willingness to project power beyond its borders. By the 18th century, the military capabilities of the army had been almost irreparably damaged. The costs of governance were only growing, this meant less money to keep soldiers equipped, let alone develop more advanced tech. Corruption amongst the banners has also been cited as one cause of military decline, which was greatly accelerated by the campaigns. Military leaders were often accused of wasting state resources and neglecting their duties to train and maintain garrisons, to what extent this significant I cannot say. However, as with most things, the macroeconomics of governance and trade are arguably more important in understanding the decline of the Qing and its army during the 18th century.

In their 2013 paper titled Taxation and Public Goods Provision in China and Japan before 1850, Tuan-Hwee Sng and Chiaki Moriguchi argue:

"Theoretically and empirically, we show that the extraordinary geographic size of China imposed increasingly insurmountable constraints on the regime’s capacity to collect tax and provide essential local public goods as its economy expanded. It is our conjecture that this factor alone might have been sufficient in derailing China from a path of sustained economic growth even in the absence of Western imperialism."

In short, demographic shifts were the biggest causes of the economic decline of the Qing. It's a conclusion that Pomeranz echoes (although with a stronger focus on the availability of coking coal) in his seminal work The Great Divergence.