advice for setting up new mac for "retirement".. NO transfer of old anything.

I am having trouble creating a new apple id.. have tried several routes but always get tangled up with having to put in my old one and am afraid this will allow my info to "leak" over from the old data..

That's correct. If that's really what you want to do, then when you set up a new Mac, and you are prompted to either sign in to an existing Apple Account or create a new one, choose the latter. Nothing will transfer, not any more than someone else's Apple Account can transfer to that new one.

Apple makes it quite impossible for one person's Apple Account information to transfer to another's... and as far as Apple is concerned, "you" = "your Apple Account".

So, something needs additional explanation about an inability to create a new Apple Account. That's primary.

Erase that Mac according to Erase your Mac and reset it to factory settings - Apple Support, and start over. That will return the Mac to its original state, without any configuration.

To create a new Apple Account Create Your Apple Account and do not use any information from any previously created Apple Account.

Correct!

That's the reason I prefaced my reply with "if that's really what you want to do." It sounds as though that's not really what you want to do.

"You" = "Your Apple Account"

If you want a "new you" then ditch the "old you".