It has no parity, you can pair with snapraid but thats snapshot parity and not real-time parity. Depends on the use case if that would work or not.

Also no caching options.

Can it access a file without spinning up all disks in the array?

I haven't used ZFS in like a decade, but would strongly consider going back to it if it can do that now.

It can't, you lose space efficiency if the disks you add aren't the same size as the old disks.

Not really with the same flexibility.

You only get usable capacity of the smallest disk in a vdev or you have to add a new vdev with your newly sized disks.

Unraid lets you mix and match however you like and get all the usable capacity (as long as your parity is your largest sized disks).