Pillars is tactically substantial, but a lot more forgiving in the character building and min/max department.
Retreat isn't really a thing. But at least "save early, save often" covers the same ground.
The bear encounter on the post-tutorial map is deadly, and the bandit camp in the East or Southeast area is unusually hard as well. The game isn't too rough on Normal from there on, as long as you don't explore too far ahead and try to do quests in lumps that are roughly the same as the order received. (I'm only midway through the game, though - and I hear the White March can be rough)
You may or may not choose to hire one ally, but more than that is highly unnecessary due to the game's providing you with one of almost every class fairly early on, though some are later than others.
IIRC...
Never joins you: Barbarian
Joins in The White March: Monk
Joins late: Rogue
Joins mid-game: Druid, Cipher, Paladin, Ranger
Joins early-mid-game: Chanter
Joins early: Priest
Joins super-early: Wizard, Fighter
It's nice to have a Barbarian, too - but you may as well make it your main character rather than a hireling. Monk is take-or-leave, and might be too fragile for the hardest setting. Ranged Rogue is a strong choice for low-micromanagement party. Ranger is sort of meh, a second-tier DPS with a third-tier tank companion... Just doesn't seem worth it in a big party.
I generally like to have one per-rest character (Wizard, Priest, Druid) and the rest use alternate resources or per-encounter abilities. I go with Wizard as the per-rest character because of early availability and because it gives me something to do with all of those tomes. Also, you get to choose whether to burn everything or try something more subtle. A single per-rest character lets you keep one stack of aces up the sleeve without suffering too much for using AI heavily otherwise.
For the tank line, Fighter used to be the best, but Fighter's Regeneration is now available for other classes. If you keep Eder around, then a second fighter or a Barbarian or Paladin is a good idea. A front line of 2-3 is good to have if you want a full party of 6. 4 would be fine too, but then you're looking at a Monk or a duplicate of one of the other classes.
If you like doubling up on classes, then of course you can hire more of whatever. I'm guessing they will write up a Barbarian for White March Part 2, but we're not there yet. For now, main character Barbarian is the unique simplest way to have one of everything without hiring extras.