Your original request was like this:
download the "FlatPak" as a file that I can archive. I then want to be able to install from that archive so that I'll have a repeatable installation process
Appimage KiCad already exists and meets your requirements above and you don't even have to install it,? you can just run it as a self contained single file.
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https://sourceforge.net/projects/kicad-appimage/
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I tried it, it works.
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