Btw Nick - thank you for the description and Juanito's link. I don't want to appear ungrateful.
But don't be too quick to throw out 3rd party utils as being unhelpful. How many of us burnt our first DSL with proprietary Roxio?
What Ventoy is suggesting is that when placed into the grub-2 compatability mode, and not being able to boot - like /every other dd method, whether from a real-command line, or with a 3rd party util / there may be one of two things:
1) There is an actual problem with the iso
or
2) It has become non-standard for post 2010 machines.
That is what I'm looking into. Not that I can't build my own.
We know that multibooters, which put their own bootloader in front of TC, differs from "single purpose" utils which do NOT do that, so we don't want to get them confused with the likes of Etcher, Rufus, etc, or even commandline dd itself.
Every single multi-booter, which substitute their own front-end loader (unless support for TC has been withdrawn), can
succesfully boot the 64 bit iso. Be it those that actually mount and rewrite to physical stick (like multibootusb 9.2), or like Ventoy which boot iso's directly, and do so in ram rather than mount and rewrite to another filesystem on the stick.
Every single "single purpose" dd'er wrapped in a gui shell,
cannot because they don't substitute their own front end bootloader - they simply deal with the iso as is. Ie Etcher, Rufus or the myriad of others, including simple dd on the commandline itself.
Again, the quest is not to build my own, but to find out why everything that dd's on the planet can't boot post-2010 uefi machines with the current iso. Well at least those who haven't accidentally declared their machine uefi-only, when in fact some are not and have csm features baked in.