Ext4 filesystem on the USB SSD.
I did have the core on the hard drive of the NAS while I was trying out the software and when it woke from sleep Roon worked fine.
Once I decided to keep using Roon, I backed up everything, deleted my RoonServer files on the hard drive and installed the SSD as per How do I create a shared folder on my external SSD, please help me! (QNAP TS253A)