Thanks. I have to use my old iMac to restore the new one again. Not falling for that trap again Past this second one. Time machine would not work and my old iMac is running off an external and that fell over trying.Might not be the method for you, but my transfer strategy - whether to a new Mac or to a clean install of a new OS - has always been to manually move selected items over from an external backup. Many of my files are on an external drive anyway so that is just plug in rather than migrate.
iCloud does its thing for certain files and data, I drag my iTunes files as the main import (getting this pointing to the right place can need tweaks), and some selected folders/data from Application Support and Preferences.
I set up all my email accounts fresh. This is where a challenge can be as I’ve found I need to export each mbox individually after first taking them out of any folders and then import them to the new Mail (I only keep mail filed locally).
Caveats, I’m running High Sierra on a 2012 iMac so it has been sometime since I’ve had to do the above so my method may not be wholly up to date, but I’d suggest moving individual chunks from Library in stages rather than just dump the whole Library from one to the other machine.
clean slate I think, and as you say, in parts. Probably only the application support for the programs I use most. quite a substantial backup plan in place, no data lost.
edit. Ah, sweet , now my old iMac won’t boot.