The best way to get your scenario, instead of being the delay or prevention of the Tiananmen protests, would instead be the total or partial success of the Tiananmen protests, which may require Wan Li's cancellation of his trip to Canada as the latest possible POD. After the trip, the student movement was doomed to fail. Thr best way to make the Chinese military or simply the PLA more promising would be a partial success of the student movement, because a delay makes mono difference, while a prevention is borderline ASB.
The delay of the Tiananmen protests makes no difference, when it is after all going to explode. The PLA today won't be much different as sanctions would simply follow by after the massacre. Clearly you aren't willing to explore the possibility of a partial or student victory even in such a scenario. That means, a delay of the protests simply make no difference,
If your POD is after 1987, that the protests won't take place was borderline ASB. There death of Hu Yaobang was simply the immediate cause of the protests. While the Tiananmen protests were largely unprepared, students and professors did plan a giant protest on May 4th, 1989, which was the 70th anniversary of the May Fourth Movement well before the death of Hu Yaobang. If your POD is before 1987, then it may have to involve a different direction of Gorbachev reforms or that Gorbachev never came to power. However, such a change doesn't possibly fit your criteria since it's likely that the whole Eastern Europe did not begin political reforms.
Alternatively, the non-existence of the Political Reform Office in China (which was headed by liberal Bao Tong) or less liberal university campuses but both later scenarios would require an even earlier POD. However, even that won't possibly lead to the total butterfly of a huge student movement that would inevitably come. Not after the Democracy Wall Movement in the late 1970s. Before that? It may have to involve Hua Guofeng's staying in power, but then the Chinese military won't be as promising.