Thank you for your excellent question and well laid out. The bun you are targeting is traditionally fairly high in sugar, and quite high in fat. Both are ingredients that create excellent shelf life keeping qualities for softness. We can see the sugar level is high in the ingredient listing.
The additives you listed I have done my best to try and line them up in your post. Our text editor for the forum does not accommodate tables at this time unfortunately. In the baking industry these additives are often generally referred to as treatments. Each has a role to play and sometimes there are additional interactions to be aware of, that are either beneficial, or can create additional challenges. On the BAKERpedia website you can look up these ingredients to learn more details about them here: https://bakerpedia.com/ingredients/
The amounts recommended to you look reasonable. I would generally start on the low side and test one at a time to understand impact and effect. Xanthan gum here is to trap moisture and retard starch retrogression. If you add xanthan to your dough, even at 0.1% bakers percent, you will very likely need to increase water 1 or 2%, maybe more, depending on your equipment.
In my opinion, I do not believe the TangZhong method would be appropriate for you needs. It makes a nice product for home applications, but the high water activity in the finished product may go moldy easily, even with preservatives added, or you would have to use so much preservative it would not taste nice.
I hope this helps, and please do let us know how you make out with progress.