I've got Ubuntu 22.04 lying around that still update because they are LTS. Ubuntu has a well publicised policy for releases and you will have obviously read them.
Try do-release-upgrade.
You also mention "OS supplied from a vendor with extensive customization. Apt was dead."
Try do-release-upgrade.
You also mention "OS supplied from a vendor with extensive customization. Apt was dead."
How on earth is that Ubuntu's problem?