Yes, but you can only mount it from one client at a time. So from my point of view you combine all disadvantages of local storage with all disadvantages of networked storage.
In case of the game installations: why not keep them local and use an automatic mechanism to backup to the ZFS based NAS via SMB, then also automtically snapshot that backup?
Get rid of all spining drives! (the vibrations in your van WILL kill them even if they are powered down.)
Also, try to use the least possible connectors. ( computers are consumer grade devices with this amount of vibrations not even considered at design time. This means also to avoid using caddys or slotted fixtures. Everything should be screwed in place as secure as it is possible. Buy quality sATA cables, the cheap ones are prone to break over time and cause annoying issues. (Slowing down the array or some drives to go offline)
Dont cram your system (As I see the HL 15 is a good start for this) it will make your cooling loud and complicated. (it is so difficult to even find ANY cheap, silent and effective 1U cooling solutions.)
Use ECC RAM, and use as much, as you can afford! TruenAS has a really not the best Hypervisor IMO, so if you want to use ti as one, you must assign the maximum amount of RAM to each and all of your VMs, in order to work at all (I dont know, whether the ballooning bug has already been patched, if yes, sorry about the incorrect info) so your 32 GB single stick will run out really really soon. I would say that this 32 GB is even the minimum that you should assign (leave to) TreuNAS itself.
Regarding the CPU/MoBo: As you can read in all my comments, for my current goals the Chinese X79-X99 motherboards with LGA2011 socket (Sandy bridge, Ivy bridge, Haswell and Broadwell) are the best price/performance units. You can get them nor really-really cheap, the motherboards are practically brand new (only the Northbridge chips are recycled), the CPUs got dirt cheap in the last year or so (I bought an E5-2697 v2 for like 30 EUR a month ago). Xeons have a ton of PCIe lanes. Aliexpress is a great place to start looking for them. Motherboards are available in various sizes, (single, dual CPU setups, From Mini ITX up to some real huge ones. They however lack IPMI or built in VGA for debugging, but TrueNAS does not need any VGA output during normal operation. X79 systems use DDR3 ECC (most MoBos up to 128GB capacity) X99 systems use DDR4 ECC (usually up to 256GB capacity). DDR3 ECC got also dirt cheap in this same last year (you can get 128GB of it around a 100EUR, from used Samsung modules.
Technically yes, but there are some workarounds to allow you to use the same HW behind to host multiple drives. Jeff from Craft computing I think has a video about it. (he likes second hand enterprise nVidia acceleartor cards as VGAs for remote gaming PCs.)
What disadvantages does a “local storage” you think there is?
If he is accessing the system alone, I cannot see any (besides the backup times due to bandwidth limitations.
iSCSI is the way around the limitations of “remote storage”.
Also for example, Steam does not like smb shares for their game library a lot.
If he plans to use that game library alone, then it could be a good solution, but built in Windows backups will be a pain in the lower half, through smb.
I have multiple computers, and almost ALL of them fails to successfully finish a backup to a SMB share due to network drop outs.
If you do it on your server, everything is done locally with maximal bandwidth.
So the big question is whether he wants to use it alone, or with friends or wife?
Also, it is all about money too.
NVMe drives are not cheap (over say 4 TB) and I havent seen yet a 8+TB NVMe M.2 drive before.
Building such a system needs a lot of thinking, and planning to be cost- power- and perfomance effective.
And could be done in several ways too, so I would not say, there is one single solution to this specification.