I have to grin. A very rational approach. Welcome back!
I’m really not sure that you will be hanging out in either Bastion or Torghast very much, though.
Assuming you’re primarily looking at world content, you will be shuttling through all four main zones a lot as you level and to gain Renown and gear from world quests in the early days. You may choose Kyrian as covenant, but they send you all over the place.
At this stage of the expansion, you will get your basic gear, up to ilevel 190-200, pretty quick, and unless you’re running M+, your next stop will be Korthia for ilevel 200-230-ish, and there is not much point spending time anywhere else.
And so to Torghast.
The ONLY purpose of Torghast is to grind materials to make your Legendary. Now, how many Legendaries do you need? You can use only one at a time, so unless you are min-maxing for different Mythic bosses or high M+ key affixes, you need only one per spec you intend to play.
Now, the amount of Soul Ash you need for a maxed Legendary is just over 5K. You get 860 from every Torghast run at Layer 8 and above (which you can read as every Torghast run once you have done the introduction), so you will need about 6 runs of Torgasht total for that. If you’re doing two leggos for two specs, double that to 12 ofc, but that’s all, for this patch at least. (We should be able to wear two leggos in 9.2, est April 2022) That’s not a lot.
Now, if you go for the Rank 6 legendary, you will also need 1650 Soul Cinders, also from Torghast. How many runs is that? You get 150 per run at Layer 8, and 350 at Layer 12, so somewhere between 5 and 10, but you will already have picked up at least 900 from the 6 runs you did for Soul Ash, so you really need only a few more.
And that’s it. Once you have that, you’re done with Torghast forever. Unless you just want to collect them all even though most are useless and you can wear only one at a time. And did I mention the price of each piece of the Rank 6 Legendary Base items? For example Shadowhast Helm https://theunderminejournal.com/#eu/argent-dawn/item/171415.262 is about 50K on your realm. So collecting unnecessary Legendaries is an expensive hobby.
As for which class, there’s a lot of opinion around that. I’d suggest checking out YouTube for examples and opinions. Healers and Tanks have been given a certain about of tuning in Torghast, which most seem to agree makes them especially viable.
Personally, I’d go with Prot Paladin, but I haven’t played many classes in Shadowlands, and especially not in Torghast (I’de prefer a dentist’s appointment), so that is a very limited view. I haven’t brought my DK into Shadowlands yet, so I hav no experience on that.
Please ask again if you have any more questions!