"Looking at SSD" tells us nothing that useful. What *exact* model SSD? SATA or NVMe makes a huge difference.
SATA drives in the internal bays only run at Slow SATA II (e.g. 3G not 6G speeds).
I have no idea why you have Pro Tools installed on a separate drive, it's just not a good idea.
We have no idea of how large any of these drives are or your budget so can't really give you much help.
If it was my system I would remove all the storage and install a few Samsung 970 Evo M.2 SSDs (
https://www.amazon.com/dp/B07BN217QG...ing=UTF8&psc=1) on PCIe slot adapter cards. Ideally switched adapter cards that deliver full speed to the PCIe 3 based M.2 SSDs. Like this
http://www.xlr8yourmac.com/feedback/...ro_Review.html and see
https://forums.macrumors.com/threads...-ahci.2146725/
I generally recommend Samsung SSDs over OWC units. Just lots of benefits from Samsung full stack ownership of these drives, e.g. the chips and firmware is all theirs to fix if there are issues.
Also depends on your PCIe slots. What slots do you have available?
With the super fast PCIe NVMe SSDs you can put everything, boot/system, Pro Tools, sessions, samples on one drive. If you need more space then sure start by putting say samples if they are large on a dedicated drive (hopefully another M.2 NVMe drive). Until Mojave it was not easy to use/boot from these M.2 NVMe drives, now it is, you now have storage options not even worth comparing to a HDD.
Samples on a fast SSD... what that might do is let you run more VIs if your VIs use streaming samples, or let you start sessions faster if the samples pre-load. Depends on the VI behavior.
For the low low cost of these super fast drives, it's not worth my time to even think about it. Unless I had huge sample libraries or large video libraries I would put everything on fast NVMe SSDs.