There is no ‘make up your mind’. There is only a building of evidence. Unless you’re willing to declare some objective standard for what everyone considers an ‘official Democrat’…
In the meantime, many people register into a party falsely to do things like manipulate primary votes and such. A common attack vector for the first would-be assassin as I recall. To say “he is or isn’t” is pretty much in the eye of the beholder and we’re discussing points undecideds will consider relevant.
My mind is currently leaning to ‘way too many people will think he is a Democrat for this to be good for Democrats’.
I’m no longer as certain as I was, but 19 donations over 5 years is a strong argument making me think he is.