To fit all currently assigned code points within 18 bits is easy: you would only have to move one range. Above 32FFF, all assigned code points are within E0000 to E01EF, which fits in-between 32FFF to 3FFFF with room to spare.

Those code points are used for flag emojis and for selecting uncommon CJK variants. If you don't support those, you could just strip out anything that doesn't fit in 18 bits to begin with.