A little friendly advice, if you'll take it. You shouldn't dismiss things that you know nothing about, especially things that are very straight forward and meet your needs exactly.
I believe SFTP is safe for the transfered files, but I want protect my nas to be accessible for everyone on the internet. I hoped there was a simple solution for this. But you guys, have that much knowledge and asking questions I can't answer. It's clear to me now that the "simple" question I thought I asked has a much more complicated answer as I hoped for.
I have been running a chroot'd (jailed) sftp server continuously here on Linux since 2001. There's even an OMV plugin for it. As for the users who need to access your files, all they need is Filezilla, but there are other choices.
Of course I'll take advice from you and other forum members. That's why I asked the question, for advice and help.
I am grateful to everyone who has taken the time to give me advice.
So I've 2 options now.
1 is the vpn option
2 is the SFTP option.
I think I'm gonna try them both and see what's the best option for me.