Hope you get it sorted mate

These sound pretty obvious but just in case

have you checked that the idle screw adjuster on the throttle body has not fallen out as this is quite common.

Also are you using the cable to alter idle revs, as the cable adjuster should only be used to take up slack in the cable.

If I remember correctly the best way to sort idle is to disconnect iscv when cold, get the engine ticking over then reconnect iscv if the revs rise then disconnect and turn idle screw out a few turns and try again until the revs no longer rise

It sounds as if your iscv is held open far longer than needed, resulting in high revs once warm which is why when you disconnect it that it then idles fine

Another sympton of this is that even when cold and the iscv is disconnected the engine ticks over and does not die which is should do if the iscv is not operating

You may already know all this but thought I would mention it just to be on the safe side and cover all angles.