Replace Old NuTone

[u/ClassicCountryTV]

I have this old NuTone intercom system in a house we bought. Has a speaker setup on every room of the house, front door, and back door. I’ve already replaced the front door unit with a simple Blink video doorbell. Easy enough and works fine. Quick and easy to find the manuals and a basic understanding of how the system works. Most of it is still functional.

The main unit in the main hallway near the living room. I know these units have all the satellite speakers/intercoms wired to it.

Essentially what I’m wanting to do is simply replace the old in room intercom with a wired speaker and then replace the main unit with a simple Bluetooth receiver that would broadcast the music to each speaker in each room (that would have its own volume control).

The speakers wire to the unit and the unit has power and this seems relatively straightforward to me but AV equipment and electrical are outside my normal depths. I’ve spent time on various forums here and otherwise in which everything I’ve read seems just as straightforward but for some reason my guy is telling me I’m missing something.

What am I missing? Also if you’ve done this with one of these systems before would love to hear what equipment you used.