So I played around with it a bit more.
I do understand why SE designed it like this. If the oath gauge was a mechanic like "on damage taken" it would reduce the gauge into nothing more than a fancy cooldown on boss fights and they might as well revert to 3.0. Since Sheltron gives you MP, this new mechanic also introduces some really fun decision-making like self-heal vs. mitigation vs. DPS/MP management. If you're willing to give up some DPS/healing you also can make your Sheltron timing very flexible now by choosing when to charge your gauge.
All in all it feels very polished and I like it a lot for levels 64-70. It really does work extraordinarily poorly from levels 52-63 now though. It's too bad SqEx couldn't have added onto the Holy Magic Mastery trait so that Sheltron simply changes from a cooldown to consuming gauge at level 64 like how some other jobs got traits that introduce their gauge. It doesn't matter for players who have remained subbed through Heavensward, but new players or returning players who do old content (or anybody who does Susano-o at min level) will find Sheltron very annoying to useless in Shield Oath and may not stick it out for the fun mechanic SE has made for max level.