Truenas L2arc
Prisc Chandola
If you can setup l2arc from the command line, you can just click the import button in the zfs plugin to use it in OMV. So, I don't think this system is out of the realm of OMV but it will require a bit of command line work. bcache would be more complicated to use with OMV but is still possible. I would stick with whatever you are most familiar with.
To be honest, I'd not even contemplate moving away from TrueNAS, it's been brilliant apart from that one unforgivable flaw of random reboots that seem to affect quite a few Supermicro implementations. The TrueNAS forums have offered little to no help for multiple related issues with Supermicro watchdog reboots.
My next plan of attack is to either move everything away from a BSD based NAS to a Linux/Deb based NAS (OMV) or to try and get some traction in the BSD community to create a bug report and hope it gets fixed but the latter may take months or years and I need it working now.