IMac or Nas

OP, generally the results are "expected" and "reported" to be better when using an external storage device (versus the internal one you are considering). The only way to find out is to try the various options within the confines of your own system / setup. 

There are advantages to be gained from galvanic isolation and other noise suppression / resonance controls for the external storage device. You lose those options / opportunities by going with your internal ssd - Roon Core iMac (all in one) solution.

Something to consider is to walk away from your iMac as Roon Core and to get a dedicated Roon Core option. 

All the best.

Tank you 

I would keep the music on the NAS.  The iMac will sound the same playing from an external server or its internal HD-I know because I have done both.  Having the music on a NAS will probably add flexibility, in that other players are more likely to recognize the NAS vs the iMac