If anyone is interested, I made some improvements [1] when using this for ISP programming.
It should work notably faster (minute -> seconds) and has some ergonomic improvements like setting chip size without reprogramming and some additional commands for exploring flash.
I have also added some reliability improvement (IMO), but it may be more or less reliable for you.
I have also added some reliability improvement (IMO), but it may be more or less reliable for you.
[1] https://git.sr.ht/~monoidk/spiflash