PsychoWards

At least for Truenas, you can backup your config and store it wherever you like. In the case of a boot drive failure, you would just reinstall truenas, reimport the config and you are good to go, no need to do an image of the boot drive itself. This will not work with hexos, since hexos will create new pools and wipe the drives. But using Truenas instead would be an option to recover the data from the drives until importing pools is possible in hexos.

I cannot help you with the Immich migration, however currently it's not possible to create a pool in Hexos with 1 single drive, you need at least 2 drives so that Hexos can create at least a mirror. If, however you want to expand your pool in the future you need at least 3 drives during the creation. Those are the current limitations of hexos, this will however change in the future. So your current plan of sacrificing 1 ssd for the pool creation will not work.

Are those WD Red or Red Plus or Red Pro? Simple WD Red are not ideal for a NAS, despite WD marketing them as NAS drives, since they a SMR an not CMR drives, the performance can suffer. Other than that, if you can spare the extra capacity lost go with a RaidZ2, meaning you will have 8TB storage and you can lose 2 drives without losing any data. However, at the moment Hexos will always create a RaidZ1 meaning you can only lose 1 drive but have 10TB storage.

And Vaultwarden if that's possible

How do you want to do this? There is no reliable way to measure the consumption of most of the parts, some parts are not even read be the motherboard (HDD consumption, PSU losses etc.) Your best bet is going to buy a cheap socket power meter, this does it way more reliable, you know how much is actually pulled from the socket and depending on the model you can connect it to Home Assistant for example.

@The Callorhinchus I'm not sure if you got an invite already but @Adster mentioned in one of the categories, which you see after you got the invitation, that the everything is running fine when installing Hexos as a VM in Promox, so at least there's that. I guess most people using proxmox just use truenas directly, the Hexos target audience probably doesn't want or need the increased complexity/possibilities? I'll use proxmox since Hexos currently doesn't support all the applications which I need and I will spin up dockers in a VM in proxmox.

I'm interested in this as well, is it better to use Proxmox for the VM stuff and use Hexos as a NAS only or should we just use Hexos for both the storage and VMs?

Yes please, this should be done rather sooner than later.