Shield Oath makes Sheltron useless.

[Furious]

You're gating Oath generation in Shield Oath behind one skill and one RNG proc. The former you get at lv64, the latter you can't manipulate without involving a 3-minute cooldown (Bulwark) or a 2-minute cooldown (Passage of Arms).
Which is nice after you hit lv64, but when you add any new system, you introduce the player to the basics of it and then add more skills/mechanics tied to it. The way Oath was implemented does not do that, since there's only two skills in the entire PLD kit that interact with it prior to getting Holy Spirit: Sheltron and Intervention. And as per the design you can't count on using them much since Oath gauge generates very slowly under Shield Oath (unless lv61-63 shields have insanely high block rates) until lv64. Hence my comment on it.

What I'm getting at is that growth does not flow very well, because a new player that never had pre-SB Sheltron is getting an ability at lv52 that they'll almost never be able to use, and will likely wonder why they even have it for the next 12 levels until they get Holy Spirit and Oath generation in Shield Oath goes from UI garnish to a functional system. So instead of being able to learn how to make use of Sheltron upon acquisition (like you do with new skills in any well-designed system), you're waiting until lv64. The same also applies to intervention, but that's a really situational ability since you'd get the most bang for your buck using it on your co-tank in EX/Savage.

Sure, everything works at lv70, but feels counter to the idea of the player learning how to use skills as they acquire them, and that just leaves a bad taste in my mouth.

The problems you are outlining are literally non-starters; they are imagined problems. People don't learn to play their class until they have the abilities in totality; the same is true of a great many classes and abilities in the game. When you get sheltron at 52, you will likely spend a lot of your time questing in sword oath, where you generate charge at a rate of 5 per gcd nearly continuously, and will have regular uses of the block available. In dungeons, when you use shield oath, you will have less, but you also don't need any.

If they were to increase the rate you generate shield charge in any other way, it would neccesitate reducing the oath generated by holy spirit, which is the entire point of the new system; a cyclical rotation between physical attacking for ~75% of the time, generating some minor charge, then supercharging with holy spirit, then back to generating mana with physical attacks.

The whole argument that people learn to play their classes as they level is and always has been a ridiculous one. Classes and jobs (in FFXIV and in any other mmo) simply don't play the same at low and high level. Learning to play as you level means learning habits that don't fit with the ability package as you get higher - you develope muscle memory for abilities that later become moot or situational, for example. PotD reinforces this; someone who takes a step into a solo 50+ potd with a new class and spends 10 minutes reading and sorting their abilities will be much more likely to have a congruent, logical button layout for their abilities than someone who put them into place as they leveled. People fail at their jobs in potd, but certainly not at a rate higher than they fail at leveling roulette dungeons or anything else that requires levels be "earned".