Does Trump believe his own lies?

You are not.

Don’t sell yourself short. You’re a bullshit artist.

this sounds right - and dementia on top of it

A liar has enough respect for his audience to try to make something sound believable. A bullshitter doesn’t care.
If you call out a liar he will backpeddle and make excuses. A bullshitter will either ignore you or double down on his bullshit.

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Trump is Chunk from the Goonies, minus anything good about Chunk

In the opera Das Rheingold, one of the characters asks, “Are you malicious, or merely insane?”

This question can be asked about those we deal with in life, and it is always best to assume they are malicious.