I'd rate Henry Maudslay's invention of the three plate technique for producing extremely flat surfaces (popularized by Joseph Whitworth) as the most important step toward modern mass production. All precision machining ultimately depends on accurate surface plates, so without them interchangeable parts are impossible.
See the excellent book "Foundations of Mechanical Accuracy" by Wayne Moore of the Moore Tool Company for details.
I learned that in 1751, Vaucanson made this full-metal lathe: https://artsandculture.google.com/asset/metal-turning-lathe/... that was a kind of breakthrough and the ancestor of modern lathe, one of the first precision machine tool. Add that to his automata and his invention of punch cards to program looms and I am now realizing how much of a Da Vinci that guy was.