Blackview BV6000 rooting problem / spyware removal / SP Flash Tool freeze
I ended up reading this question after I also tried basically all of the steps / websites mentioned in the question. But I eventually managed to install TWRP.

In order to make the SP_Flash_tool start flashing, after you plug in your phone that is turned off, press power button + volume up

If the flash tool errors, you might not have the right scatter file or twrp recovery.img



If you are not sure about scatter file, try flashing a stock rom (I simply clicked on the latest) and follow the PDF mentioned in the link and you'll be able to use the scatterfile from the downloaded rom.
After flashing the entire rom, it took quite a long time to boot (7 min ish) but the effect was as if nothing really changed. I had backed up my phone before flashing this stock ROM using the stock recovery so now is the time for factory reset, then restore data, then install all updates, re-enable again developer mode, usb debuging and oem unlocking and make sure that the updates actually got installed successfully and that there are no more updates.

Then try this TWRP (12.1 MB (12,783,616 bytes)) which also contains a scatterfile but that scatterfile didn't work for me, I had to use the one from the stock rom that I had just flashed. So in case the flashing still errors at this stage, I think you can tell that its because the version of TWRP is not right, not the scatter file.


Note: The file sizes are of the actual unzipped recovery.img