The Soviet Union was never an expansionist project in the military sense (they wanted to spread the revolution abroad, such as by assisting the Republicans in Spain and giving weapons to the Vietnamese in their anti-imperialist struggle)

I don't think this distinction mattered to the capitalists. Whether we're talking about military expansion or about supporting a socialist revolution in Germany, the capitalists didn't want it to happen and Hitler could serve as bulwark against both. Had he kept to his own borders, Britain and France would've been perfectly satisfied with that result. Instead, because he invaded Poland, a country he, again, would have had to go through to reach the USSR, they declared war. I really want to emphasize and repeat this point: If Britain and France wanted Hitler to invade the USSR, what physical route was he supposed to take?

The fact that all of these western leaders talk of the USSR using the Molotov-Ribbentrop as an “odious but necessary defensive measure”, proves to me that they understood that the USSR wasn’t something they needed to be militarily defended of by a weaponized Germany acting as a buffer, hence that can’t be understood as Germany’s role in the situation in my opinion.

But, as you mentioned, the Soviets had supported the Republicans in Spain - even if they were too vulnerable to launch a military invasion of Germany, there was a possibility of them supporting a revolution in Germany, and Germany of course was politically unstable. The capitalists already had their "win" of the German communists being defeated and the class conflict appearing to stabilize, that's plenty of villainous motivation on its own.

It seems completely implausible to me that they wanted Hitler to invade but then when he started moving his borders closer to making that possible, they suddenly flipped out and did a 180 and declared war instead.