Sorry not answering your quetion but that sounds like a complete nightmare to me.

Any specific reason to use “Nextcloud”? You can always just use SMB or something along that.

Zerotier or Tailscale is for you. I personally use Zt but the recent update with Mullvad support really empowers Tailscale.

Yes.

Again, it’s just a markdown. You can do this:

![desc](path/to/file.mp4)

Cannot recommend Obsidian enough. Just plain Markdown, but infinite times more powerful. I use it for just that and it’s been damn perfect.