Do you mind my asking what Processor is in your Mac Mini??
Also, IMHO if you are committing to using it as your Roon Server, then it would definitely benefit from upgrading the Drive to SSD
You could add a 2nd drive to the Mini, but Roon has to run on the OS drive I believe
The Roon Database with all the Metadata etc., is kept on the OS Drive......and as such Roon would benefit from such an upgrade
Samsung 850 Evo SSD's 250Gb on Amazon for £78.......with 500Gb being about about twice that
With 250Gb being more than enough for Roon and OS, this would be a very significant upgrade for £78 IMHO.......most likely more than you would get from the extra 8Gb of Ram you speak of above, unless you have a VERY large number of albums........even though extra Ram never goes astray in the long run IMHO
Food for thought!