Really interesting article and I loved the work and extensive research you have put into this article, trying to progressively look at a better fit by looking at the R Squared value. As you rightly pointed out, the more we try to curve fit the actual price, the more the model is prone to out of sample testing errors!
@muratkbesiroglu what model do you think is most suitable for pricing Bitcoin. What do you think of the Money supply equation. I read an article where the author used it