I’m not sure what’s going on with your power calculation there. Edit: It should be the Hubble constant times length times whatever tension you let build up in the wire from the generator’s resistance to movement. Planetary masses are only significant as your contraption gradually starts the endpoints moving.

Even mithril will have to break before it’s moving faster than lightspeed. Otherwise, sure, fill your boots. If you set up in a gravitationally neutral area, it should start noticeably moving at around a light year of extension. You’ll need way more to overcome the escape velocity of the Earth, sun and Milky Way, though.