When is lvl skipping add on is justified? (if ever)

[Niwashi]

My problem is mostly I'm too concerned about my lvl. I want to enjoy the story but I don't want to be gated and have to grind for it. I'd rather grind first then I can enjoy the story till the end. I was thinking getting 1 job to lvl 70 will eliminate that requirement.

It will. And it sounds like you've answered your own question. If the level gating really bothers you, then eliminating it would help you, and you can go through the story without worrying about it.

There are some obvious tradeoffs, of course. If you're doing level 30 or 40 quests on a level 70 job, any non-level-synced combat will be completely trivialized. It's hard to create a sense of danger or accomplishment if you can one-shot the enemies with an auto-attack and nothing will aggro you. Even level synced solo instances will put you several levels above the one they were designed for, making even the normally challenging ones quite easy. Do you have an active enough imagination that you can picture your character facing off against a fearsome beast when the difficulty level is more akin to stepping on an ant? If you do, then you're fine getting a level jump and then just going through the story. If not, it could make the story seem pretty flat, trying to build up excitement over things that end up trivially easy. (This concern really only applies to open world or solo instanced content, though. Dungeons will be done mostly with over-leveled but level synced players regardless of whether you're one of them or not.)

You'll have to decide which issue is a bigger concern to you, level gating or over-leveling. (When deciding that, keep in mind that the overall leveling pace of the game has sped up quite a bit since 2013 and 2014, leaving fewer places that you'll hit a level gate while playing through the story than you would have gotten back then.)