More importantly, how long will my 3.16.9 continue to operate before it struggles with the aeronautical and map data downloads?

It's impossible to answer your last question. We take backwards compatibility seriously and tend to support old versions way longer than most companies do, but there will always come a time when doing so becomes more expense than it's worth. It's normally a "many years" timeframe though.

I shall use the situation to demonstrate to my household Financial Controller that a new tablet is required.  Which would you recommend between an iPad Air or a Samsung S8 Plus?

There's a thread on the PilotAware forum where some people have had problems with unstable connections between device/SD and PAW  after upgrading their Samsung device to another with Android 15.

If you are a PAW user, it may be worth looking more into this if you are considering a Samsung.

The question - is there any imminent threat of my SD simply ceasing to function (beside normal hardware reliability factors)?

I did notice some 3rd gen iPad mini's (which share the same form-factor) and go up to IOS 12,  do you foresee the build tools at some point not supporting this,  is there a timeframe?

There's no imminent of your SkyDemon simply ceasing to function, but there's a good chance that over time your old software will stop understanding the new ways we're doing some things - such as new ways of transmitting aeronautical or scenery data - and some parts might not work so well. At that point, it really would be best to treat yourself to a brand new device which will hopefully give another ten years of service.

I believe that by the end of this year iOS 12 will be our minimum, but that might stretch into 2025 a little.