No, Rage is only replaced when you enter the Supernatural Fury
The way this is worded, the Rage feature is not removed upon reaching level 3. Instead the feature is disabled while in the Supernatural Fury. This means that you could enter a "regular rage" but these features are disabled while in the alternate state.
This shows that the wording is probably flawed
Because of the earlier bit in the feature: "You gain the ability to cast and concentrate on spells, even while raging", you could also cast spells while in a "regular rage." This is almost certainly not the intent.
Its viability depends on external factors
It is mostly viable, actually the only viable ranged Barbarian build in my opinion1.
The external factors are:
- party composition
- available space
- encounter type (if it is a single boss, or many smaller monsters)
- enemy type (if it uses attacks)
When I played this build, it was quite useful when everything was optimal.
However, its usability decreased significantly with one of these factors missing.
Party composition
As mentioned in the comments of this answer, this tactic is only good if your party members can keep the target of your Ancestral Protectors (Target) away from you.
Either they have to physically hinder its movement, or discourage it with threats of Opportunity Attacks.
You will want to have at least half of the party members to be in melee.
When the Shield Master fighter was not present at our encounters, my Barbarian worked a lot worse.
Available space
If you fight in a dungeon, sometimes you just can't get far enough from the target while maintaining line of sight.
Encounter type
If there are lots of enemies, you might get far enough away from the Target, but his comrades still might be adjacent to you, causing disadvantage on your attacks. You have to hit your target to trigger Ancestral Protectors, so this can be a big problem for you.
Enemy type
If the enemy does not use attacks, only breath weapons or spells, Ancestral Protectors become irrelevant. Unfortunately the resistance is only against his attacks.
Conclusion
It is not viable for dungeon crawling with a party of 4 Sorcerers, but with the average party in the average encounter you will do more than fine.
Recommendation
Use a Hand Crossbow instead of a Longbow and multiclass to Rogue instead of Fighter.
Hand Crossbow
With the Crossbow Expert feat you can attack without disadvantage even if you are adjacent to enemies, negating the worst consequences of a tight space and many smaller enemies.
When you already have the feat, Hand Crossbows give you the most DPR .
Rogue
2 levels of Rogue make Disengage a bonus action, very valuable in tight spots.
Also a Rogue Multiclass provides better DPR increase than a Fighter2, and this area is where you are very much behind a usual Barbarian, as you can't use Reckless Attack and the damage increase during the rage.
1 Being at range gives you extra survivability, by making it harder for others to target and hit you. Ranged characters come away from most fights undamaged in experience, so resistance is mostly wasted on them, and no other significant Barbarian features are usable at range. Other ranged Barbarian's usefullness in combat is below that of a Ranger, and they are famously weak.
2 A single level of Fighter might still be a good idea for Archery fighting style.
Best Answer
Ancestral Protectors would not affect a target you hit before entering a rage
You can take bonus actions before actions in a turn but this does you no good because your bonus action, activating rage, would prevent you from casting booming blade.
If you instead cast booming blade first, which includes making an attack as part of casting booming blade:
You get to the heart of your question. Ancestral Protectors says:
Most people would probably read "hit" and understand it in the subjunctive tense. That is: you must first be raging and then make an attack for the Protectors to take affect.
However, out of context, the sentence also makes sense if you read "hit" in the past tense. That is: when you activate rage, the Protectors affect any creature that you already attacked that turn.
The problem, if you use this second interpretation, is that the past tense would not make sense after the first turn when you entered the rage. You can't be talking about a triggering attack in the past tense when haven't made an attack (say, at the start of subsequent turns). So you have to understand the sentence in the subjunctive mood and that means the attack must come after the rage is entered as a bonus action.
So booming blade won't work with Ancestral Protectors. Further, the Protectors would only affect a creature you hit with an attack resulting from action surge if your normal attack action produced no hits.