I think you’re quite right that Reagan was not an isolationist.
Reagan’s genius was never having to bring our military might to bear. Our enemies understood that we had the capability to annihilate them, and believed Reagan might just be crazy enough to pull the trigger. Reagan leveraged that threat of annihilation to win the peace.
Huge divergence from the current wave of never-ending limited engagement conflicts.
I think that Reagan would have acted just as George W Bush did after the worst attack in American history.
In 2001 Reagan would not have had to devote so much of his massive military buildup and manpower to the European theater.