Again: Change AppleID to .icloud from 3rd Party

Hello!

There are several topics but i didn't find a suitable answer.

I have a 3rd Party Email Address for my AppleID (gmail).

Now i want to switch to a .icloud Email AppleID.

I have an old alias with .icloud which is also mentioned under "contactable at" with the (?) next to the Email under account settings.

I also made a NEW alias under .icloud.com Email Settings on my AppleID (gmail) account.

When i want to this NEW Email alias into my existing AppleID:  "You can only add the @icloud.com address that is already on file in your account."

When i enter this NEW alias at "contactable at" i get " This email address is not available. Choose a different address."

I can enter an other (new) .icloud Email that i generated before to "contactable at" and the i have an (x) next to it with the possibility to delete.

The other ones (gmail and old Alias) has a (?) next to it (this is your apple-id, this is your alias).

My question: WHERE do i have to have the NEW Email (alias) that i can change this to my new Apple-ID?

I am really lost... Thanks a lot for your help!