I'm not really sure what you are saying but it would be rather OP if you could just spend a couple of hours in an end grinder and then stock all the XP up and never farm again.
It's taking away a core part of the minecraft process no matter how it's implemented, levels or XP.

someone could also just use those levels right away,...

yes i guess it would make the game a little easier to play implying not having to grind for those levels over again no matter what EVERY time they die, or use all there levels, but then again people with alts could just post their alts in there grinders and leave them there to collect the exp orbs and use just the levels from there alts to enchant items, not giving them the face of reality of possibly losing their levels

also you wouldn't necessarily store the levels, only pull out the levels,...you'd be storing the exp orbs as the bank is scaled like a players levels, and would take only the highest levels out of the bank at each use

so idk how storing the levels is so much more op implying what i said earlier, "taking 1 level out of the bank while the bank has 150 levels stored in it would only give you 1 level not the way OP approximate 100 levels" (that taking out the orbs of the 150th level would give you),and that you would have to spend the amount of orbs as it would take to get from level 149-150 to store that level, and that you would have to have at least as many levels on you as in the bank would take away from the bank, so just using the 1st level without having the first 30 levels to instantly skip from level 29 to 30 wouldn't be possible, however if you had 30 saved up in the bank then you could take the 30th level, reducing your bank balance to 29 levels, and use it but then would have to stash more credits and levels into the bank to use it again for the purpose of going from level 29-30 instantly again because they would need to have to store their 30th level again and then pull that same level out again. also saving up to an insane amount of levels would just be a waste implying that you would have wasted about a ton of possible levels that could have been used storing up from levels 120-130 (if you were to use them right away you would be able to get from level 27-30 at least 40 times over, but would only gain 3 more books to enchant by taking out at the mass of 130 levels. Being at higher levels to enchant also gives no real advantage either)

you would still have had to use the same amount of exp orbs as it would have taken to get to level thirty in the bank and it is truly more efficient to just simply use the levels when you hit 30 rather than save up, cause the higher level you become the more orbs you are wasting to get from, lets say, level 70-71.

if you would make people vote to use it (spend 5 crafty credit) then it would also limit peoples use of it but give them another reason to give this server votes. people would limit there use of it but it would help new-ish players, or just bad survivors, or players who don't have grinders, to be able to enchant a few items on their own before they would acquire a grinder.

-excuse me not writing more or in any more detail right now, i got to go and am not going to be able to revise this until a later point in time today