OK so i've changed the i2c bus back to a level shifter for each data line (4 2xSDA and 2xSCL), I thought the HC4050 would act as a sort of cluster of level shifters. But now I see why it works for SPI and not I2c. The flash works perfectly on my other project and it did work at 3.3V as that's the voltage at which the teensy 4.0 works at, but i'll check the Datasheet for the W25Q128FV.
Also with the MISO MOSI they all connect to the same bus so using the HC4050 should be ok in practice? I'm not too sure why they won't benefit from the HC4050.
I'm creating a data logging flight computer for a model rocket, so I'll be logging data multiple times per second, so I need as much power/speed as I can get with this processor. (I'm using this as purely practice in terms of creating all SMD components and wiring the board from scratch as I'd like to create my own fully designed flight computers from scratch in the future) 
Thank you again - Harvey