Just replaced the hd in my MacMini late 2014 with a SSD (torture) the difference in the sound is fantastic - major upgrade in SQ. Also moved my source from NAS to ssd in USB 3 enclosure directly connected to the Mac. Initially very disappointed as the SQ and volume were very poor. Plugged it into another USB port and WOW fantastic!!
Just red your message, my understanding is that you installed an internal SSD for OS and SW and connect an external SSD via USB for your music library ?
Why not use your internal SSD for both SW and music library ?
When optimizing, it is good practice to avoid USB links in order to reduce noise (mainly jitter).
Your NAS can then be used to backup your SSD , a RJ45 link is known to be less noisy than USB.