Possibly, DFU (How to revive or restore Mac firmware – Apple Support (AU)) may be the solution. Main problem is that you need another Apple silicon Mac but an Apple Store might be able to do it.
Next steps when I have the bandwidth
– I could try Safe Mode boot up, but I would not really know what further repair steps would be, I don’t think I have any “third party” apps that might be the issue.
– more likely: do a target mode boot of the defective drive via a system running Sequoia and run a Reinstall macOS from the backed up Mac install dmg. If a Reinstall macOS doesn’t work, I could erase the whole volume to install from scratch.
– When originally engaging file vault, I perhaps misunderstood the need for a Recovery “pin” Recovery Mode options only recognizes only a 3-group pin sequence, not password for the drive. Booting into Options Mode, will allow password to engage in any attempts to repair, erase, or install on the volume. I’ll need to boot from a Sequoia system, or lower, of some sort in order to reinstall Mac OS 15
Good Luck with all that! Sounds like a reasonable plan for an Intel Mac on Mojave. My advice to get Sequoia again would be:
-make and boot from a bootable USB Sequoia installer
-Delete the existing Sequoia volume pair using DU booted from the USB
-Still booted from USB, create new volume on internal, preferably in same container as Tahoe.
-install Sequoia onto this new volume.
-migrate in from your Sequoia backup.
Final thought…do you really want Sequoia? sounds like you are reasonably OK with Tahoe as I am, so why have the hassle of dual booting?
Re: the hassle of dual booting
Indeed, at this stage, this would be a curiosity, pending bandwidth to engage in the endeavor.
p.s. If I were to proceed looks like I would need to make the M4 accessible to an older computer via a “share disk” mode as an Optional boot method. Unclear whether this sharing would actually allow installing software … outside of my experience/familiarity
…and mine. And not the way Apple intends for dual booting to be done: