It has been asked many many times. Unfortunately blackview is not going to release source codes, that's because of an internal policy. I'm sorry.
So we will never see a lineage OS or another cfw for this device?
never say never... but not for now.,
I saw this video about the porting of and Android ROM...
Here the link: https://youtu.be/8ZKOo50bo48
It is possible try to convert the ROM without source code?
The only requirements are: a backup of your stock rom and a custom rom of a device with the same Mediatek version, in this case MT6757 I think.
So if it is possible I could try it!