As has been stated, the role of Counsellors of State have been etched in stone for such a long time, we're now seeing how archaic the role is and how it needs to be amended and brought into the 21st century.
It made sense (say 100 years ago) that the ones closest to the top of the line of succession (and the monarch's consort) were the ones to step in when needed. These were the people that most likely were closest to the monarch and had the king's/queen's ear on things day to day. They *could*, when needed,, determine best what the monarch would do or say in any situation that arose while they acted as a Counsellor (or regent).
With today's technology and the world shrunken to where a burp can be heard around the world within seconds of it happening, even with the monarch out of the country, with secured phone lines and secure Zoom calls, the monarch can "take care of business" no matter where he/she is in the world today. Even through a global pandemic, the Queen was able to fulfill her daily tasks as Queen without interruption. She still met with her PM albeit virtually and the red boxes continued without fail.
Today, in the present climate, instead of worrying about Counsellors of State and who they are, as it is more in line with keeping up a tradition that has been around for a very long time, I believe another option would be taken. Should Queen Elizabeth become unavailable and/or incapacitated and unable to fulfill her duties, the best move would be to actually name Charles as regent and have him step into the role of monarch in her stead.
If the roles of Counsellors of State do remain, I do believe it should be a requirement that those that are among the ones eligible should *at least* be active working members of the "Firm". In this case, both Andrew and Harry would be ineligible solely because they aren't in any way associated with the working mechanisms of the "Firm" and the monarchy itself. One can't decide on how a monarch would act on matters of state if they're not even included in the day to day activities of it at all.
JMO as always.