Well, I'm pleased to say that the new WWW/XMPP/MQTT NUC server is up 
and running, although I still haven't made the commitment to switch of 
the old Raspberry Pi 3. One reason for this is that I didn't yet have 
a backup of the new server: it took me nearly a full day to set up and 
there was a *lot* of software to tweak. Of course, I run a daily 
backup of important files and directories (and who doesn't eh?) but as 
there's a lot riding on this server I normally run a full disk clone 
using Clonezilla. Believe me, there's nothoing quite like being able 
to hot-swap a drive over in the event of problems...

The other reason for waiting was that I read a few reviews of SSDs and 
learnt that althought the Corsair Force LE200 is a good device, it's 
slow. When you've been used to a Raspberry Pi, "slow" is a relative 
term so the disk seemed okay to me, but then I read about it being 
targetted at first-time laptop replacement scenarios, and I was 
somewhat concerned. I tried to find an alternative and had hoped to be 
able to get a WD RED 120Gb SSD, but such things don't exist, so I had 
settle for as WD Blue 250Gb SSD instead. 

Next day delivery's a wonderful thing and I duly set up another disk 
stress test, with the following results over 12 hours: 

  Bytes read:       6517728 MByte ( 250.56 MByte/s)
  Bytes written:    3336576 MByte ( 194.88 MByte/s)

2019/12/06 04:28:20 PASSED with no errors

Wow! That's a lot faster than the Corsair on write! 
 
  Bytes read:       3348540 MByte ( 226.54 MByte/s)
  Bytes written:    1768228 MByte (  62.10 MByte/s)

So while the web server will mostly be running in read mode, the other 
applications will be writing more and anything that can be done to 
speed this up will be welcome. 

If the new SSD works as anticipated I'll switch over the web server 
next week and repurpose the Raspberry Pi 3. The Pi-hole server could 
do with an update, so that's sorted! And just in case, I've ordered 
*another* WD Blue SSD which will become the new clone backup. Great 
fun eh?!