You can buy those monument grills pretty much everywhere, home depot, Lowe's, Walmart. I don't know if you can see one on display in all those places, but they sell them. At first glance they look very similar to the newer Weber grills and are reasonably priced. As with all those off brand grills, I would be concerned with customer support and parts availability 5 or 6 years down the line.
All true but most of the generic, universal, adjustable, burners and flame tamers and igniters are available either from Amazon or often in the retail stores you named. So if the firebox and frame are robust enough, you can keep those grills going for years with some basic maintenance. Replacement ceramic IR burners might be more of a challenge to locate though.
BBQration 5-Pack Extends from 11.75" up to 21" L Universal Replacement Heavy Duty Adjustable Stainless Steel Heat Plate Shield, Flavorizer Bar, Heat Tent, Burner Cover, Flame Tamer for Gas Grill
https://www.amazon.com/dp/B0785JHFDG/?tag=tvwb-20
ADJUSTABURNER Gas Grill Burner Universal Stainless Steel Tube (Pack of 3) Extends from 14" to 19" BBQ Replacement Parts for Nexgrill, Brinkmann, Dyna-Glo, and Most Gas Grill Models
https://www.amazon.com/dp/B09B4HS9R2/?tag=tvwb-20
Grease Tray for Gas Grill, Universal Drip Pan for 4/5 Burner Gas Grill Models from Nexgrill, Dyna Glo, Expert Grill,Kenmore, BHG and More - Stainless Steel Grill Replacement Parts (22"-30")
https://www.amazon.com/dp/B0BLYS7NSF/?tag=tvwb-20
Hisencn 6 Outlets Tact Push Button Chrome Coated Universal Spark Generator Switch, Grill Ignitor Kit for Home Depot Nexgril 720-0830H, 720-0830D, 720-0888, 720-0888N and Most Nexgrill Grill Models
https://www.amazon.com/dp/B08R8MRF29/?tag=tvwb-20