I read through the article. A lot of it is above my head. I guess I initially went with the Airport base station with two, possibly incorrect, assumptions. One, the Apple device would be more secure than the Verizon-provided wifi and two, it would be easier to connect my Apple stuff to. It's the first assumption that is troubling me now as the Airport has not been updated in almost two years. I like the idea that the double NAT setup is more secure.
So, to summarize:
o Is it risky to continue to use the Airport given the lack of updates?
o How secure is the Verizon router if I just use it i.e. any history of issues?
o If I go to another double NAT setup, which seems preferable, would any particular brand be more compatible with my Apple devices? Any recommendations for a relatively low-cost unit? Please note: speed is not a big issue for me and range equivalent to the Airport would be acceptable.
No its not risky, the disadvantage of the AirPort Extreme is not its security, but that it's not the fastest all in one router, that's all. It does have decent range. Where speed and what you use becomes a factor is with higher ISP Mbps connectivity to your WiFi router. By that I mean you have a 200+ Mbps connection when connected using ethernet versus a Wifi Router that you see only 50 to 100 Mbps when wandering around your house on your computer/table/phone using WiFi.
see
Verizon FiOS Router and Security Issues - Verizon has had a few major security issues they needed to release updates or patches for. If you using a ISP provided WiFi router that does everything, then its only one wall to scale for to access your devices on the router's price IP address connected devices. The below diagram shows how much easier it is to circumvent one all-in one modem/router/wifi security versus a second router with WiFi
Double Nat allows you to use just about anything for your second router or mesh setup, its what you been using all this time. It also means that your second router connected devices are behind a another range of private IP addresses. Most people use this configuration, like you are doing now with your setup.
On what to buy, as seen by some of the
Apple Store examples there is more to a smart purchase than a single router versa various Mesh solutions. A single router could be just a Netgear Orbi (RBK50) if your Airport Extrem died. Why did I mention the Netgear Mesh again, because for a single unit it speed vs distance coverage for a house is good. That only $149, #4 in below article.
See
Best mesh routers for 2021: The best mesh Wi-Fi systems
You can of course use many traditional wireless routers with lots of antenna sticking up too. Just need to know you have many options to read up on.