Debian 12 will not boot: Screen flickers between frozen boot messages and black

I have Debian 12 (updated to 6.1.128-1) running on a Thankpad X1 laptop (kernel 6.1.0-31-amd64) that stopped turning on after routine system updates (apt-get upgrade) on 2025.02.09. Boot messages appear, but freeze after a time. The screen repeatedly flickers between black and the now-frozen boot messages. The exact message it stops on varies, with Starting NetworkManager-dispatcher.service being the current example that I can reproduce. I conjecture that it's trying to bring up graphics and crashing immediately (and then repeatedly), but debugging this is a bit beyond me at the moment. I have kernels 6.1.0-30-amd64 and 6.1.0-31-amd64 available and they both display this behavior. I can boot into recovery mode, and can get the wifi working in the resulting root shell, but not much else. I tried removing nvidia packages on a lark, but this changed nothing. Normally my next steps are to do a complete reinstall, but I'm hoping there is something I'm missing and there's a way to rescue this?