Easiest and most secure way? Mail (or hand deliver) a flash drive. That's how they transfer data between super computers and data centers. (AWS even has dedicated trucks to do it)
This makes me wonder, what is the difference in the environmental cost of uploading/downloading this data vs. shipping a USB.
I would guess that shipping emissions would be higher than digital ones, but I don't have any basis for that theory. (I'm just curious, not trying to say or imply anything here)
I tried creating my own torrent and was able to dl it on another device, but on her machine it stayed at 0% and wouldn’t let me connect to seed
At least one of the torrent clients needs to be fully connectable (port forwarded) for torrents to transfer data. You need to test that e.g. test your torrent client's incoming connection port with a port test website like https://www.canyouseeme.org, https://www.yougetsignal.com/tools/open-ports, etc. & make sure those port test websites can successfully test connect to your torrent client's incoming connection port. If the test fails then you need to look at opening the port via your OS firewall and/or router firewall.
Is FTP a good option? I set up a proxmox server last night but I don’t really know what I’m doing yet
Probably best to avoid FTP if you don't know what you're doing, it's not all that secure.. you'd want to at least configure SFTP or FTPS which is just going to be more complicated vs fixing your torrent issues. And technically you still need to make those connectable (port forwarded) too, just like your torrent client.
All that aside it's probably easier to use Syncthing if you can't get the torrent working.
You could also try one of those file transfer websites that use WebRTC to transfer data peer to peer e.g. https://file.pizza or similar. Not sure how well they work for huge amounts of data but their github page mentions that Firefox is better for that, apparently Chrome starts to choke with data 500+ MB.
Btw: Might be that you're behind a NAT (router) and that's why bittorrent doesn't connect. You'd need to figure out which port your torrent client is configured to listen on and then do "port forwarding" of that port to your machine in the router you got from your ISP. Or use something like UPnP that does this automatically.
Not sure if that applies in your case and it's unsolicited advice... But a fairly common issue with bittorrent.
Toffeeshare is pretty good for this kind of thing. I don't think there is a size limit, only restriction is that it must be only one file so you must create an archive to send.
But no install, no configuration to do, it's one of the simplest way to send a file IMO
I'm assuming both of you can connect to peers on other torrents, just not each other. Again, do you guys have DHT/PEX enabled? Did you accidentally create a private torrent?
Maybe check out Tailscale. It's mainly a mesh VPN for your own devices, but they have a lot of options included so you can share stuff with other people.
I've set up tailscale in the past week and fallen in love with the ease of use. So, this has my vote too. But, if i was doing this, i would chop the file into, say, 500mb parts using 7z or WinZip and then transfer it through SCP (WinSCP if using windows) over tailscale IPs.
Maybe create a VPN connection with wireguard, then you can just transfer them however you'd do it in a local network? Tailscale would be an easy solution to achieve this.