Job balance doesn't matter before max level. The biggest reason for this is that there's so much variability in gear while you're still levelling. Getting a beneficial dungeon drop can produce large changes in your performance. The other, perhaps more salient point is that most people either don't care about how well you optimise for Sastasha Normal, or they lack the knowledge to assess this.

The main goals of the levelling process should be to:
1) Progressively introduce you to the mechanics of the job
2) Allow you to form a decision (hopefully early) about whether it clicks with you or not

Levelling is the tutorial. Core steps in your rotation should be introduced early so that they become second nature. There's no point introducing the "3" in 1-2-3 at level 70+. I know that the dev team likes to introduce these in late as the "new ability" for an expansion, but this fools nobody. (What, you aren't excited to see Malefic 19?) No, adding another combo step that should have been there all along doesn't absolve you from having to come up with a few unique abilities every two years. Hire more than four people to manage 17 jobs, yeah? Oh, you squandered the money from this golden egg into your other (less worthwhile) projects? Thanks.

Likewise, shorter recast abilities should be introduced before longer recast ones. Abilities like Sheltron and even TBN should be introduced before abilities like Rampart and Vengeance, even if only in a self-only form. Knowing that a cooldown will be back up again in a mere 20 seconds time makes you less afraid to press it. It also gives your tanks more agency. Hey, I can do something to stop myself from dying at this part! That's huge.

It's important for tanks to highlight playstyle differences early. If you wait too long to introduce in each job's unique resource system, the player is forced to grind through an unnecessary amount of content to realise that PLD/GNB's current "resource" systems are less interesting than watching paint dry and the real gameplay is a paint-by-numbers rotation. But, hey, spreadsheets might be your thing, I'm not one to judge (I am judging you).

Also, why would anyone want to wait 60+ levels to use a gap closer? I do understand the need for progression (save your big offensive cooldowns and invulns for the very end). Even still nobody wants to slow walk through the first dungeon that they queue for, just because they haven't earned the right to jump from pack to pack by wading through hours of story monologues. Bring out the mobility tools and unique fun stuff for each job early, and add in the big damage moves with long cooldowns as the icing on the cake at the very end of the levelling experience.

If you want to see how to streamline levelling, just look at Warcraft's pre-Shadowlands patch designs. They've pretty much got it down to a science.