"Supply and demand" does not refer to your definition of "supply". The "supply" in "supply and demand" is the number of bitcoins available for sale at various prices. "Supply" is not "the amount of bitcoin that is being dumped".
When people refer to the supply of "new" bitcoins, they are really talking about "production". "Supply and demand" is a concept. "Production and demand" is not.
There has been only one bitcoin halving so far, so I don't think you know what you are talking about when you write "halving has always had a big impact". Furthermore, if the you look at the last Bitcoin halving, you will see that nothing happened immediately after the halving. If you look at the last Litecoin halving, you will see that the price fell due to the speculative bubble but has remained fairly flat for nearly a year.