Jumping into a frying pan. Orbi connectivity issue and router meshing with orbi?

You raise several interesting questions.

• Orbi and "single ID".  This is correct.  Orbi broadcasts one WiFi name (SSID) on both 2.4G and 5G channels.  Devices decide where they will connect.  There are some internet standards regarding how to choose, but devices vary considerably in how (or if) they implement them.  Devices designed to "roam", such as laptops and phones, often will switch from the router to satellite (and back).  Devices intended to stay in one place often attach to one access point and remain stubbornly on it.  You have hit one of the most common questions on this forum, "WHY does my *** connect to the wrong place?"
• "AC1200" is a description of the "Theoretical speed" of a WiFi system, not a model number.  Please describe the product name rather than the speed.  For example, Nighthawk R7000 or Orbi RBR50.
• Yes, you can attach your existing device directly to the Spectrum router (over Powerline if you want).  That will set up an entirely separate WiFi system that is not integrated with the Orbi.  With a 1,000 sq. ft. addition, this might be a really good idea.
• One of the things people on the forum are sensitive to is having a "Double-NAT" (Google it).  That is, one router behind another router.  It works (I did it for over a year, and my guess is that 1,000's of other WiFi owners do as well.) But, there are specific situations where having two routers in a row is a receipe for disaster.  People post, "I can't get this (or that) to work", and the solution is to make one of the routers "no longer a router."