don't give up. try rkdevtools. the problem with it I cannot help you much (because I am not yet familiar with the Rock64 way of doing it) is what is needed to enter that MASK ROM mode or how rockchip calls it. This link talks about how to get into that mode having FW somewhere. for SPI you need "to short spi signal to GND". On my settopbox the job was done by holding some magical "fw upgrade button" at the power on for some time. 
On Rock64 I am sure there is the way too. rkdevtool utility connects to the board and hopefully is able to cammand rom code to erase that freaking NOR flash.
On Rock64 I am sure there is the way too. rkdevtool utility connects to the board and hopefully is able to cammand rom code to erase that freaking NOR flash.
ANT - my hobby OS for x86 and ARM.