Consider coding the bat form from the ground up using only vanilla commands.

Requirements :

1. Any damage dealt to the summoned bat should be repercuted onto the player multiplied by 10/3 (max health of player divided by max health of bat).
2. The player should not be able to use any tools/weapons while in bat form (maybe anything).
3. The player should be resized to the size of a bat (to fit in 1/4 block spaces).
4. The player should be immune to sun damage while in bat form.
5. The player should emit some bat sounds while in bat form.

Additional Information :

1. Version 1.20.5 - 23W51A contains command "/attribute @p minecraft:generic.scale base set 1" that allows to scale player models