That's not even the type of setup you should use. Use a VPN of the type designed for games and IoT stuff, like ZeroTier, n2n, and more. Then you set up a local file share using something like Samba, only accessible by the people who can connect to your local network via the VPN.
The public facing VPN code will be MUCH more hardened against attack than your typical sharing tool with port forwarding.
ZeroTier can be a fully self hosted VPN. You set up a server locally, port forward only the VPN service, and then everything else you run is accessible through it for the people you give access to.
ZeroTier is pretty easy to set up, but at the point where you're worrying about "barriers to sharing" you should probably using a cloud service anyway.
Ahhh but that's the thing. A middleman being necessary is very against my values. I don't want there being someone else or there limiting or telling me no. I'm not letting someone else's DMCA compliance tell me what I can have. I'm also not really interested in non FLOSS.
You should be able to just use ssh/sftp. There are lots of great clients, and you can absolutely still use usernames and passwords, no public/private key stuff required. You can even use ssh and scp right from powershell on Windows boxen if you're so inclined. There's winscp, and if you want filesystem mounting, there's this: https://github.com/winfsp/sshfs-win
For macos and Linux, the options are far more plentiful.
Edit: there's also file pizza, which is a file transfer thingy with no middle man that's open source, although it's not copyleft AFAICT: https://github.com/kern/filepizza
and similar tools. Not really what you're after, I just think it's neat.
There are definitely a lot of good options out there. What are you using right now for regular old FTP? The odds are actually pretty good that it already supports SFTP. A lot of file management applications do both and lump them together, even though they're completely different protocols (sftp is from the late nineties).
If it doesn't, then I don't know what OS you're using, so I'll just recommend options for the big 3. For Windows, there's WinSCP. For MacOS there's Cyberduck. Most file managers on Linux distros let you just type sftp://me@wherever in the navigation bar, meaning you get a totally seamless experience with the rest of your FS.
EDIT: or, you can use sshfs-win on Windows and have your remote filesystem show up as a regular ol' drive, just like SMB. MacOS and Linux have sshfs, and I know there are GUIs wrapping sshfs on those platforms. I personally use sshfs at home and it's great (although no GUI wrapper, I'm a weirdo who doesn't use a graphical file manager at all).
:( I'm sorry to hear that. Well, for Android there's MaterialFiles, which is fully FLOSS and supports FTP, SFTP, and SMB. Not sure about iOS, but I imagine there are options there.
I hope that your journey through life becomes a little less rocky.