You're factually 100% correct. Mulund is the last Station before Mumbai ends and Thane begins.

On the Western side, Borivali is the last station before Thane district begins with Mira Bhayandar, before Palghar starts with Dahisar and Vasai-Virar.

Idk how people are considering Dahisar as part of Mumbai. It's not even on the Mumbai (Salsette) Island. It's literally part of Bassein (Vasai) territory and yet people on here will argue for hours to make Dahisar part of Mumbai.

By that logic, not just Thane City but Kalyan and even Ulhasnagar is part of Mumbai. Lol. It's not.

Thane, Dahisar, Ulhasnagar, Panvel, Navi Mumbai, all the rest of the areas which are beyond Mulund and Borivali are part of Mumbai Metropolitan Region (so technically still Mumbai City on a global scale), but not actually part of Mumbai, since they're not officially considered Mumbai district.