v5.12 mentions C1 again, this remembered me to give a short update here.
Last thing I planned was to compose a working mainline kernel from pre compiled stuff at kernelci.org.
Their config is here, modules and zImage are here, dtb here and rootfs (possibly) here.
But as a replacement I had to take my spare C1+ into "production" ad hoc so there is no test platform here at the moment (OPI+2E ordered).
Hint: v5.10 mentions
but the C1+ isnt, there is no working docker storage driver for it.