I’m looking forward to getting an Apple Watch with cellular service to be reachable although a dumb phone would be way cheaper now that you mention it

Lol, as if Apple would let you off that easily!

The watch can't have a cellular plan of its own. It must be associated with the plan that's on the iPhone it's tethered to. If you use a dummy iPhone for this purpose, then the SIM for your primary number needs to be on that phone.

The carrier charges 5 € a month for the Multisim service