As per my first post in this thread, I recommended a Tomini Tang for the OP tank.
@GrandmaRaff
I certainly make my share of keeping too large of a fish in a small tank back in my early days keeping marine aquarium fishes umpteen years ago, but I learned from my mistake. One only gain experiences if one choose to learn from mistakes.
I have keep Yellow tangs, Powder Blue tang, Hippo Blue tang in my 65 gal. I do have a 320 gal tank and these tangs were moved out of the 65 gal tank with time, usually less than 1 year.
I ordered a PBT online, he arrived thin, and sick. I have him in my 65 gal tank ad nursed him back to health. He really grew fast and was moved to my home tank.
On arrival, less than 2 inches on 4/22/2018. Now he is conservatively estimate at 7 inches.
6/24/2019
4/19/2020
at 7+ inches in a little more than 2 years
A friend broke down his tank and put a Yellow tang and a Hippo blue tang in my 65 gallon aquarium. Samll about 2.5 inches. Initially, the Blue Hippo tang did fine but he grew quickly and start to stay under the rock just as you, and many people notice, all day other than go out to eat. Unlike you, I see this change in behavior as a sign of stress and gave him to a friend. I don't like Blue Hippo tang so I did not bring him home to my home DT. IMO (only my opinion) while the color of this fish is nice, I don't like the jerky movement of his swim motion, and don't think of him as very graceful. He is doing fine. The Yellow tang also go to my friend becasue I already have a Yellow and Purple in my tank at home.
My fish are fat, colorful and healthy. I am a small and not a muscular person, 5'3'' weight 135 lbs. I really hate to haul water so I set up my aquarium so that I don't have to haul water for water change. When I change water, I just turn a valve to drain water then turn another valve to fill water to my tank. All have to do with how careful you are in setting your aquarium up.
A few pictures of my Fat fishes
You have a good day