Sounds good. A few thoughts:
Mud for refugium: I would ONLY put it in an easily removable/replacable container. If you look at any of the guys who promote using mud, they ALL replace it entirely every year or two. I would just use some plastic storage bin for it and leave it in your sump, then toss it out in a year and refill it. I think "old tank syndrome" really means "all of the phoshate has been bound to my rock, sand, and mud, and there's nowhere else for it to go except the water column, where it's free to be picked up by algae, so algae explodes everywhere syndrome".
I'm still on the fence about efficacy of mud, but if nothing else, I do believe it is a phosphate sink.
Originally I was going to say no to the naso tang because I was thinking unicorn tang, which while in the same family technically, the naso should stay a bit smaller. 5' will be pushing it, but that being said, in the 3 years my 120g tank was up, my sailfin didn't outgrow it, despite him being a very large fish. I would start with smaller fishes if you can OR see if anyone put a naso in a biocube and are looking to rehome it now that it's outgrown their cube lol.