Ubuntu 20.04.3 and RAIDed root partition

[Leroy Tennison]

I have searched the web and what I have found (including the Ubuntu RAID guide) is either outdated or wrong.  In the installer (either the default or updated one):
If I select "Use as Boot Device" for a device then the device is not shown as a device for "Create Software RAID"If I select "Use as Boot Device" for a device and "Add GPT Partition" (to create an unmounted partition for root) then I can't select a specific partition in "Create Software RAID"
If I create a software RAID (mirroring) first then it can't be selected as a boot device
All I have is terabyte size devices and can't afford to dedicate one as a bootable device.
I'm now looking at installing Ubuntu without RAID then attempting to create a RAID of root after reboot but I can't seem to find how to do that either.
Can you provide instructions (or a URL to same) for either method?  Thank you for your help.
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