The most convenient way to deal with this on that modern machine is not to use CorePlus, but TinyCorePure64 loaded by Ventoy which easily deals with Secure-boot.
You create the Ventoy drive with the secure-boot option dropdown. Copy TCPure64 iso to it.
Reboot and enroll the keys for your machine. Once done, it will be recognized normally thereafter. See the Ventoy project for more details.
Of course, in this environment, it won't recognize the cde directory, so standard procedure is to either copy cde directory from the iso you mounted previously, rename it to tce - to a supported external filesystem, or build up from an initial commandline environment setting your tce location manually. (tce-setdrive et al)