I wonder to find case suitable for the new blackview p6000 i pre order thanks

A soft case already comes with the device

Perhaps unrelated but my question is that the USBc to 3.5mm connector was so fragile that it broke within 2 weeks.
I went ahead to buy some replacements, whether is it a USBc headphones, or a USBc to 3.5mm convertor, neither of them works and hence I am unable to use my headphones.
Can you advise why this would be the case?

Hi! This should work without issues:
https://www.amazon.de/OTB-Stereo-Adapte ... B07B8QHDSK

You can also use the USBC otg cable (wich came with your p6000) paired with a "big"USB to audio jack adapter.

By the way, I recommend to start using a Bluetooth headset, wich will make you cable (and hassle) free.