I also got myself into a similar situation by playing around with too many firmwares without fully understanding the process.
The threads that helped me:
1:
https://www.mobileread.com/forums/sho...d.php?t=254042
Especially RK 2.1 tool (first post)
2:
https://www.mobileread.com/forums/sho...d.php?t=245888
Especially point 6. BUT I used the tool mentioned above (middle tab - "Upgrade Firmware") to flash the full firmware image file.
OR if you can still use adb: "adb boot boot-loader" and then only flash the system.img with the first tab ("Download Image")
OR when the reader is switched off. Hold the refresh button in for 8 seconds and then insert the usb cable (this should get it in "boot-loader" mode)
OR on some firmware if you keep both the refresh and power buttons pressed it enters "download or boot-loader" mode.
Hope this helps...