The router was fine. That is the part worth holding onto. It routed packets, it served DHCP, it had survived three years in a cupboard without complaint. So naturally I decided to flash OpenWrt onto it, because the stock firmware had a web UI from 2018 and I have a problem.
The flash itself went perfectly, which should have been the warning. It came back up, I logged in, I started moving config across, and somewhere in the middle of toggling settings I managed to misconfigure the network so thoroughly that the device stopped answering on every interface at once. No LAN, no LuCI, no SSH. A small blue brick with a power light.
The recovery, of course, lived behind four solder pads I had been ignoring. UART, 115200 8N1, and a USB serial adapter I knew I owned and could not find for forty minutes. Once I had a console the failsafe boot was easy, and I had it back in ten minutes. The forty minutes hunting for the adapter were the actual cost.
I did finish. It runs OpenWrt now and it is genuinely better, with proper SQM and a config I can read. But the honest lesson is the old one: the thing that works is allowed to keep working, and "while I'm in here" is how a quiet Saturday becomes a soldering session. Keep a serial adapter where you can find it. You will need it on the day you were certain you wouldn't.