First, when you have a cache pool with 2 members, the amount of space available with the default RAID1 config is equal to the smaller of the 2, because each piece of data must be duplicated on a different member. BTRFS free and used space numbers are a work in progress, the display is pretty much guaranteed to be wrong.
While the CPU you have in the Unraid box is older, I don't see any reason why you couldn't run all your acquiring and organizational apps as containers on Unraid. If plex isn't doing any transcoding it would run comfortably on Unraid as well. If you are serving more than 1 client, and if any are remote or need transcoding, then you need more CPU, so leaving the current plex server would be better.
The amount of CPU needed for the 'arrs and related pieces aren't really very significant, I think it would be better to keep the traffic all in one box vs. doing it on different boxes and moving it around the LAN.