The router was fine. That's the part I keep coming back to. It sat in the corner doing its job, NAT-ing packets, not complaining, not asking for anything. And I decided it would be better with OpenWrt on it.
It wasn't a need. It was the itch you get when you read a forum thread where someone has the exact same model and an extra 30 megabits of throughput after flashing it. I told myself I wanted the better QoS. What I actually wanted was to poke it.
the flash that didn't take
The stock web interface had a firmware upload box, which is the trap. It accepts the factory image happily and refuses the OpenWrt one with a vague "invalid file" that tells you nothing. So I dropped to the documented method, TFTP into the bootloader during the boot window, which on this model is roughly a second and a half if you hold the reset button and have the timing of a session musician.
I got it on the third go. Progress bar, reboot, and then nothing. No web UI, no DHCP, no ping on the expected address. The classic half-bricked state where the bootloader is alive but won't hand off to the system.
the serial console I didn't have
At this point you need a serial console to see what the bootloader is actually saying, and that means UART pins on the board. Mine had the pads but no header soldered on, and the silkscreen was unlabelled. So there I was at half ten at night with a multimeter in continuity mode, probing pads against ground to work out which was TX, which was RX, and which would let the magic smoke out if I got it wrong.
I found them. 3.3V logic, a USB-to-TTL adapter I'd bought years ago for exactly this kind of evening, and the bootloader log scrolled past confirming it was trying to boot a kernel that wasn't where it expected. A second, more careful TFTP of the correct image for the correct flash layout, and it came up.
Total downtime: one evening and most of my goodwill toward myself. The router now runs OpenWrt, the QoS is genuinely better, and I have learnt nothing, because I will absolutely do this again. The lesson isn't "don't flash routers". It's "solder the serial header on first, whilst the thing still works, because you will need it at the worst possible moment". Buy the cheap pin headers. Tin the pads. Future you, standing in the dark with a multimeter, will be grateful.