Thanks for the reply.
I don't want to go 1:1 mirror, because quite simply the data that's on the drives is not valuable enough to invest in another 3 drives (which is what it would require) plus the controller and external enclosure that would require!
I've come around to thinking that I can afford to lose a drive if it happens - hopefully scanner will give me at least some preemptive warning, but if not it's not a huge problem.
I'm still copying data off on to a multitude of smaller drives so that I can break up the Storage Spaces pool!
The scanner already picked up 173 errors on one of those drives which I suspected was a bit flaky, so a good omen so far :-)
I can use folder level duplication presumably for say photos and home movies, plus they are also in OneDrive so I'd be covered on multiple bases there - that stuff I cannot afford to lose!
The rest of the data is media, so whilst I would be inconvenienced if I lost it, it is actually recoverable with a little time and effort.
Cheers!
Thinking about it, I think if I lost a drive, I could get the missing files info from Plex so again, not too big a problem.