Honestly what you ask for OP is more hassle than it's actually worth since by default it's something that would need to be done for each job individually and manually for each level range depending on when the job gets the upgrades. How do you compensate for utility that the on level job lacks like AoE damage on Dragoons prior to mid 40s? Or the additional utilities tacked onto Black Mage the further you go? Actually that one is an interesting case study on its own.

Let's assume you somehow manage to Equate the damage between max level skillset and at 4 different combinations depending on the level range (and for the record it's just fire spells. For simplicity sake I omitted Thunder) Hell let's assume rhat for level 15-20 (earliest dungeons) and 20-35 you somehow balance between max level Astral Flame and lower stacks. Hell let's also assume you somehow solved issues with Enochian and the Polyglot spells that aren't static parts of your rotation as they mostly fill in the blanks if you have a slot for extra GCD. Now let's look at all the things that don't have direct damage numbers on them. How you balance Umbral Ice? If you lower the mana ticks to keep similar phase ratios you essentially ruin the flow of high level BLM. If you nerf damage on AF even further to compensate for higher ratio of AF to UI you'll make BLM damage pitiful compared ro their "on level" version during shorter encounters. What about Umbral Hearts which additionally extend the Astral Flame and allow for things like Casting an extra Flare during AoE pulls? Triplecast that allows for moments of Hyper Mobility or weaving in whatever extra abilities you still have making Quadruple Flare swift and easy?