Got it about the Arris SB6183 modem. It's on top of the modem list, replacing the Motorola. So far, it seems to be widely recommended. It certainly should do the job for me. Thanks everyone who mentioned it.
Considering all the difficulty people have had in finding a stable and non-crap router, I may have to bite the bullet and pay for a Google Mesh router or two.
At present, my office is approximately centrally located on the 1st floor of my house.
My house's footprint is roughly 33' × 40'. (I got out my 50' tape measure this morning which I haven't used in a very long time
.) My office is roughly centrally located on the 1st floor. In it I keep my desktop computer, cable feed, modem, & router. I run a short cat 5 cable from the modem to the router & from the router to the desktop. I also operate a laptop computer, two smartphones, and Chromecast via WiFi. At present, we have no problems with WiFi deadspots anywhere in the house, on either floor. The basement, with steel I beams and support columns, is unfinished.
The office shares an interior wall with the family room where the HT & TV are. The distance between the router and the HT is maybe 3 feet. I could easily run a cat 5 cable from the router to the HT, but so far I haven't needed to do that. I'll try one Google router in the office and see what happens. If there are any dead spots upstairs, I'll get a 2nd router.