Sorry for waiting, I'm not so active here latest but I actually made a data pack for it, here are the commands ;)
But at first, the floating island dimension creating is the same, if you want to make it as a separate dimension, in the dimension folder you change the name "overworld" to any different name, for example, floating_islands, then you need repeating commands blocks or tick function, here's the command
execute at @e in minecraft:overworld as @e[distance=0..,y=325,dy=16] run function script:teleports/islands_from_overworld

This thing checks the Y position from 325 to 325+16 then in the tp function I have some visuals but you only need this: execute as @s in script:floating_islands run tp @s ~ 0 ~
So the command would look like this:
execute at @e in minecraft:overworld as @e[distance=0..,y=325,dy=16] in script:floating_islands run tp @s ~ 0 ~