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OK 80 GB is for sure an edge case. Nextcloud won’t even work for that due to PHP memory limits, I think.
Interesting problem. FTP is an option, with careful instructions to an untutored user. Maybe rsync over a VPN connection if it is always the same sender.
Not even sure what else would reliably work, except Tannenbaum’s Adage.
You’re describing the world wide web, except giving others write access