It is virtually identical to Earth in size, mass, and composition, and it sat in the exact same stellar neighborhood with liquid water oceans for billions of years. Yet, it crossed a tipping point and transformed into a toxic furnace.

But significantly closer to the sun.

how can we assume that they somehow triggered the changes that appear to have cause the big change?

We can't and don't.
Being a little over 2/3rds the distance to the sun compared to earth, the sun generates nearly twice the solar energy per unit area at the surface.
So one theory is the higher temperatures from that caused any water that Venus may have had turn to vapor, and then the sun's intense ultra-violet rays decomposed the water into hydrogen and oxygen, with the hydrogen escaping into space.

 

With all due respect sir NASA is going to launch venus mission in between late 20s and early 30s. The mission is DAVINCI +. Why??

Because Venus is a planetary time machine that shows us the end-game of a habitable world.
It is virtually identical to Earth in size, mass, and composition, and it sat in the exact same stellar neighborhood with liquid water oceans for billions of years. Yet, it crossed a tipping point and transformed into a toxic furnace.
If we want to truly understand what keeps a planet alive—and ensure Earth never triggers its own irreversible climate breakdown—we have to study the one world that got it wrong. We cannot fully understand our own planet's future by only studying Earth; we need to look at our twin.

More recent theories and simulations are pointing pretty strongly to Venus never having had oceans. Plus, how is being significantly closer, such that it receives about twice the insolation, qualify as "exact same stellar neighborhood"?