The default OMV root filesystem is very small. Most of it is in RAM the whole time. So it won't benefit a lot from being on a SSD. Boot will be faster, but running, not so much.
However if you use dockers or other software to have the NAS do more than being just a NAS, then there may be a lot of benefits to have that software installed on a SSD. Especially if there are databases and metadata and other similar stuff involved.
As long as you use the flash memory plugin I would suggest to leave the rootfs on the SD card and put all extra software, dockers and so on, on the SSD. And data files on a normal HDD.
Having the rootfs on a SD card or a thumb drive also makes it trivial to clone the rootfs. Just clone the SD card or the thumb drive.