You've seen the game's code and you're a coder? That's the only way you can say for certainty how long something would take to implement.

None of us outside of SE know how long it would take to add what you want to the game.

However, what I do know is that any time spent on something means time not spent on something else.

And with this, you are the only person I've ever heard asking for progress on leveling to be slowed (no one else has replied to this thread saying they'd also want something like this). So people in your shoes are likely a vast minority.

Which means on a list of things to add to the game, your request I would prioritize as very low. Because even if it is something that wouldn't take long, it is still time that the coder could be spending working on something that more people would get use out of.

So I would advise you to just do whatever you can to slow the exp gain that you can in game and/or figure out a way to come to terms with just ignoring things in game that you aren't ready for yet. Just because something is unlocked for you, doesn't mean that you have to immediately stop what you're doing and learn about it or deal with it.

Even skills, because until you start getting to end game content, as long as you're not holding the group back from progressing, no one's going to care that you aren't using every spell in your arsenal according to the meta for your level.

Because I'd put the chances of this being added to the game before you're max level at extremely slim.