Does imagining a different partner while having sex or masturbating count? I would imagine most people would say, “no”.

You can't share that though so while I still think it is immoral, it is also kind of impossible to know.

Now a highly skilled portrait artist paints a near replica of somebody he knows, but has never seen in the nude. They never mention their friend by name, but the output is lifelike and unmistakably them.

Maybe a digital artist finds a few social media pictures of a person and decided to test drive Krita and manipulates them into appearing nude.

Those would be immoral and reprehensible. The law already protects against such cases on the basis of using someone's likeness.

It's harmful because it shares images of someone doing things they would never do. It's not caricature, it's simply a fabrication. It doesn't provide criticism - it is simply erotic.

Taking that a step further, is it illegal to find somebody’s doppelganger and to dress them up so that they look more like their double?

If the goal is to look like you, I would imagine it is possible to defend by law. Otherwise, it is simply coincidence. There's no intent there.

I don't think it is a stretch or slippery slope. Just as a picture is captured by a camera, a drawing is captured by a person or a machine.

Both should be the same and it is often already the case in many jurisdictions around the world when it comes to CSAM.