Hey Tim
I’m not really qualified to go into the nitty-gritty details. I just migrated my system (around 20,000 tracks + 2 years of history) from Win10 to ROCK, and the database that I backed-up & restored amounted to over 20Gb, but maybe some of that was redundant. Either way, the migration worked beautifully, and I lost nothing in the process. Bear in mind that the database grows with time.
You might want to take a look at this post for more details about recommended hardware
Again, I am no expert, but I believe that Roon always installs to the boot drive, so adding an SSD to your Mac would require you to re-install MacOS and boot from that SSD in order to get Roon to use it for the database. Most likely you don’t want to be doing that, but maybe someone will come along with a better suggestion.