I'm not a macOS dev, so not talking from
direct experience, but you can select a minimum deployment target in Xcode as low as macOS 10.13, I believe. There must be a reason some devs choose not to support that.
When you say local files, do you mean files on a usb stick or hard drive plugged directly into the device?
I use Infuse on my Apple TV to access my videos on the network. It can pick up media from a samba share (or probably any other network share). There's more info on their site: https://firecore.com/infuse
It can also serve as a front end for a Plex server if you run that. While this isn't "local files", it works well and serves a similar purpose. Note: there is a very fair, cheap monthly/yearly/lifetime subscription required for any advanced features but it's free to try. I pay $12.99 for the yearly subscription and I'm incredibly satisfied. If this reads like an ad, it's not, I'm just really happy with it.
Can you elaborate on why you'd want to opt-out of DNS over HTTPS? I was under the impression that it was useful and good for privacy, but I may be misinformed.