Take windows calculator and divide the file size in bytes by 512.
I just did the math. My file is exactly 24,904,712 bytes.
$24,904,712 \div 512 = 48,642$ blocks.
Since my Wireshark reaches block 48643, the transfer is actually 100% complete.
However, after the transfer, the router blinks white for 20 minutes and never reboots, or it stays solid orange. If the file is fully transferred and the MD5 is correct, why is the router failing to flash it
Now switch it off, make your computer back to DHCP client, and wait for 1st boot. It takes some time without WAN, also every screen will take another minute to time out during initial setup.
And remember wifi's disabled, you need to keep using an Ethernet cable.
It is pale-orange and blue, OEM fw does not use white mix of 2 OpenWrt can.
Update: It's working!
My bad: on this model, the LED stays ORANGE even after a successful flash. It only turns BLUE after you actually go to 192.168.31.1 and complete the initial setup wizard. I was waiting for the blue light to appear before connecting, but the router was already waiting for me!
A huge thank you for your time and patience: @anon63541380, @alexq, @frollic... you guys were lifesavers.
Before I even think about going back to OpenWrt (not today, I need to check if my wife, kids, and cats are still living with me!), I need to understand what went wrong. How did a simple Reset cause a full brick? What could have happened during that process to break the firmware so badly? I want to make sure this doesn't happen again.