The router was fine. That is the part I keep coming back to. It routed packets, it served DHCP, it had not fallen over once in two years. So naturally I decided it would be better with OpenWrt on it.
The flash itself went smoothly, which should have been the warning. It was the moment afterwards, when the device came back up with no LAN address and no obvious way to talk to it, that the afternoon properly began. I had skimmed the wiki page rather than read it, and the default network config was not what I assumed. Twenty minutes of failsafe mode, a held reset button, and a serial console I had to solder a header on for, and I was back in.
Once I stopped fighting it, the thing was genuinely good. SQM sorted out the bufferbloat I had been quietly tolerating, and I could finally see what was on my own network. But none of that was a problem I actually had before I went looking.
The lesson, again, the one I never seem to learn: a box that works owes you nothing. If you open it up, it is because you wanted to, not because it needed it. Worth doing anyway. Just be honest about why.