Well my goal is to use ESXi and I'm going to give it a go. Just to let you know, I also have a router pre-configured to replace my Sophos, just turn off my Sophos machine and turn on the router and instant internet. I planned that for when I'm not home and the wife or daughter have some issue, it's an easy fix. So I'm saying I have a fall-back in case Sophos doesn't work initially as planned. Also, My original FreeNAS server will not be harmed while doing all the testing. I will use a some older hard drives and create a pool with the same shares, just a smaller capacity, but I plan to have it all running and leave it that way for 1 month or so. If there are no issues, I'm all in. The only thing that is really important to the family is the internet so as long as I can provide that, I'll stay out of hot water.
Also, I've heard other folks have done this sort of thing without issue. I think the big thing to realize is your system will be as stable is you not constantly screwing around with it. I'm sure I'll reboot the system a few times in the beginning but once I have things situated, hopefully the computer will be running without a reboot at all, well except for when I shut it down to perform maintenance like replacing a fan, dusting out the interior, replacing a hard drive. Otherwise, I have no intentions to turn it off. There are things I need to test such as a proper shutdown when the UPS signals for it, that will be key to me. Like I said, others have done it properly so I'm sure I can as well. Plus there are folks here that will be more than happy to tell me where I'm screwing up.
If you go down this rabbit hole, my advice is to not use your FreeNAS hard drives, use an older drive that you can spare for testing until you have the system running as expected. BTW, I backed up all my data from my FreeNAS pool to other storage so if something bad does happen, I'm still safe, just unhappy of course.
But first I'm waiting on that CPU and RAM to show up, tomorrow is when it should arrive, unfortunately someone must be here to sign for the package, I hope someone is here. The UPS idiots are never consistent on delivery times.