No worries at all. I've used TrueNAS before, and you should be fine with adding an ssd pool. That'll work well with a 10GbE connection. :)
There's one thing you might want to try first though, which is to create an iSCSI volume from your hard disk pool and try running your Steam library from that instead of NFS.
iSCSI is a "single user at a time" access thing (unlike NFS), but the caching acts differently to nfs so you might get a better result. Or not. ;)
Am suggesting that as it could be useful to try prior to spending money on ssds. :)
The actual mounting of your TrueNAS iSCSI volume from a Linux box just needs the installation of "open-iscsi" (on Debian anyway).
You run the appropriate iscsiadm command to log in to the iSCSI portal, then mount it:
# iscsiadm --mode node --targetname "iqn.2005-10.org.freenas.ctl:myshare1" --portal myserver --login
# mount -o noatime /dev/disk/by-label/NAME_OF_THE_SHARE_IN_TRUENAS /Games
Unmounting the iSCSI share afterwards is the standard umount command, then you log out of the iSCSI portal:
# umount /Games
# iscsiadm --mode node --targetname "iqn.2005-10.org.freenas.ctl:myshare1" --portal myserver --logout
Anyway, hope that helps. :)