Thanks! I have connected an SSD to the iMac and run a few CCC tests.
I might not go back to TM at all, CCC seems to handle things better.
If I were to resuscitate my ancient Time Capsule I would probably attach an SSD to it, it that’s an option, rather than using the internal disk. For the moment I went with HFS+, since you rightly pointed out that it won’t make much of a difference in terms of performance, and at least all my machines will be able to read it (but maybe this is only relevant if I do use TM).
@JohnAtl in this post Backups (especially of large files) - #16 by JohnAtl suggested using tailscale which looks brilliant. I will probably use it for Arq-encrypted CCC backups to a remote location (I’ll copy the bulk over manually and then go incremental), possibly with a Qnap DAS in both locations.