Should I buy BTC on 6500, or wait to possibly go even lower?
That would already be a good price since we don't know how low will it fall. Setting an value to buy is a good idea. But still, that's up to you if you will wait for its value to be lower than 6500$ since whether lower or not on that value, the thing that matters is that, its market value will eventually increase so you better invest to it and hold.
Predictions are useless because bitcoin is unexpected and buying at this moment is good move otherwise waiting is not the best option because prices are really moved to the 6.7k and it will not take time to go back to 7k plus so invests into bitcoin now and if you are sure that more fall will occur then wait for it otherwise on others behalf you cant decide bitcoin future I have bought now because fall is not possible.