Hi,
Once you have the CH341A programmer and the SOIC8 clamp, you have to dump the content of the SPI memory to your computer. I used the ch341prog tool. You may want to dump it a couple of times and check you got the same exact dump by calculating its MD5 or SHA1 checksum.
I used binwalk to inspect the different partitions in the flash dump and their content, but this step is not actually necessary so you can skip it.
To modify the bootloader, I opened the SPI memory dump with the Bless tool. The changes to be made are detailed in the first post (namely, replace the "bootdelay...off" part at 0x19690 with "bootdelay.....5", keeping the length of the text the same).
With the modified memory dump, it's time to write it back to the router, with the same ch341prog tool.
After this, the U-Boot bootloader can be stopped with a keystroke via the serial console and an InitramFS can be booted.