Following more detailed suggestions from Flashman, got it to work OK. Can scroll upwards from a button at bottom of the dynamic area to the top of the dynamic area, across more than a screenful.
Selection of target block was a challenge, but clicking alongside the top-of-bloc + sign did show Bloc info in sidebar. Entered a custom ID (default #bloc-N seemed unsuitable, it looks like N is bumped on each new added-bloc no mater what position in layout).
So now have a more refined option than ‘Scroll to Top Button’ Page Option. Can scroll to (the top of) any Bloc, perhaps even insert a dummy one for the purpose. Generic button can (I assume) be customised in size and label to suit, maybe even color.
Thanks for the learning exercise!