honestly i think the end needs a little more of a challenge. A few ideas for the ender dragon, telekinesis. the dragon will grab a player in mid air and the dragon has to be shot before five seconds with a bow or else the player will be thrown far across the island. The ender dragon can also pick up blocks to be thrown at the player.Second idea, summon ender tornadoes which sucks up players and teleports them when 10 to 50 blocks when touched by the tornado, bigger tornadoes that are stronger when the ender dragon loses health. Summon angry enderman when the ender dragon is angry. Add the ender dragon abilities that are in minecraft xbox edition.

The enderman should also have more abilities when in the end. teleport player in front of them when they become angry. Transform into angry enderman when killed. disassemble body parts and get thrown at a player, rebuilding afterwards.

The dragon is pretty much done and won't have any changes to it apart from bugfixes. Some tweaks to Endermen could be nice though.

maybe a few ideas from the ender dragon boss can be put in your other mini bosses
like telekinesis for the ender eye and ender tornadoes for the ender demon