Thanks for the replies, guys. I grabbed openwrt-18.06.1-ipq806x-netgear_r7800-squashfs-factory.img and will use atftp following that guide... seems as though the following is the recommended usage of atftp:
% image=openwrt-18.06.1-ipq806x-netgear_r7800-squashfs-factory.img
% atftp --option "mode octet" --option "timeout 60" --verbose --trace -p -l "$image" 192.168.1.1
EDIT: worked flawlessly!
% ssh root@192.168.1.1
BusyBox v1.28.3 () built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
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OpenWrt 18.06.1, r7258-5eb055306f
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