Thanks, I’ve come to realise over the years that Time Machine isn’t really designed to work well over the network. But it’s so convenient for a laptop that I’ve just accepted I will have to restart my backup periodically. The btfs solution sounds like a great work-around, but unfortunately my network drive doesn’t support it. 
As Time Machine is just one part of my backup strategy, this works fine for me, manually plugging in a drive (which I do for my SuperDuper backup) would mean I don’t always have a local backup that is up-to-date to the last few hours. And for a backup where I have a deep history of changed and deleted files, I use my cloud backup. I say this as an example of a setup where the ‘keep forever’ feature of cloud backup is useful.