The Meaning of Life, the Universe, and Everything.
I don't know about you, but to me Bottles 'o Enchanting are WAY too anticlimatic. They are so rare, and they barely give you any XP.
UPGRADE: The ability to CREATE Bottles 'o Enchanting.
Mechinism: Hold a bottle while collecting XP, and it gets collected for later instead of leveling you up or mending anything.
In order to keep it from getting too OP, maybe you only get a fraction of the original XP (which would still be better than wasting hundreds of XP for one of the higher levels)
OR, maybe you need to craft a special bottle/enchantment specifically designed for harnessing XP.
(I just thought of this, but what if the glass/bottle had to be made out of Soul Sand instead of regular sand/glass. Which would make sense, because Soul Sand already traps souls/xp) This would maybe help this be more of a mid-game strategy instead of early-game.
Finally, these bottles would also be useful with Sculk. If you want to grow sculk but don't have lots of mobs to kill and spread it, you can save up their XP and throw it later.
I've thought this recently, the mechanic I propose for creating bottle o' enchanting is a hopper above a brewing stand with water bottles in it. This way you can automatically make them. If you have carpet on the hopper that would block the xp from going into the hopper and being circulated. Also, you'd still need blaze powder and wait for them to brew up, so you might need more brewing stands for a bigger xp farm. The xp could stack to 64 in the brewing stand, but not able to be taken out as an item. Also taking 21 xp to brew 1-3 at a time, since you get 4-7 xp from each bottle, that seems fair.