I will write about this here if I end up with something I feel is genuinely useful and worth sharing.
Well, I have installed the current release of JMRI (version 5.14) onto my late-2013 iMac which runs OS 10.15.8 'Catalina', and the software seems to be working. This is just editing the roster, I don't have any DCC hardware connected yet.
I learnt one important thing along the way: the version of Java for this combination
must be version 11. The documentation for JMRI points the user to Java version 17, and while this appears to install, it will not run.
I have found my JMRI configuration files too. They are in the user's Library folder, which Apple have set as a hidden folder by default.

So I feel buoyed with success. The JMRI roster seems like a fair way to manage a collection of models, and I now have a roster holding details of every H0, 4mm and 7mm model I possess
I await a Sprog IIv4 from Coastal DCC, and when this arrives I can hopefully start managing and indeed optimising decoders.