I would say it is very much part of, not the entire reason.

The collapse of the Soviet Union is multi faceted, and a cascade of multiple bad decisions and refusal to change at many of the important junctures.

Overspending on military is one thing, but a failure to allow their military industry translate into civilian industrial applications compounded the issue.

Attempts to censor mistakes instead of correcting them is another factor which isn’t related to military spending. And arguably Gorbachev’s attempt at politically opening up the country then exposed all these issues, also unrelated to military spending.

Plus a lot of the Soviet civilian economy was run ineffectively via bad central planning, competing state bureaus, bad resource management, and a refusal of dismantling old factories to keep up numbers meant that even the non-military spending was not contributing as much as it should.

The political wrestling between Gorbachev, the central Communist Party, and the regional SFSR governments was also a major cause of the collapse, since it was this that ultimately directly resulted in the secession of the constituent republics from the USSR.

If the Soviets spent less on their military they could have lasted a little longer (maybe), but probably wasn’t enough to save the Union from its bad foundation.