Curiously, neither one lets me flash 180305, which is listed as the current version on the Canadian support portal: https://www.tp-link.com/ca/support/download/archer-c7/v2/
This kind of thing is why I stuck with the stock FW so long, le sigh.
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Oh fun, so there's a note on this page about there being separate versions for US and rest-of-world: https://openwrt.org/toh/tp-link/archer-c5-c7-wdr7500
I grabbed the archer-c7-v2-squashfs-factory-us.bin file and we seem to be in business now!