Answering on the idea of port forwarding. Though like others have said, tailscale or similar is a better option.

Opening an ssh port would allow file transfer. You could also so remote desktop with port forwarding over ssh and vnc (or some other more modern protocol, haven’t futzed with anything like that in a long time).

Probably doesn’t need saying but make sure your system is up to date. Also using a nonstandard port and using key based authentication and disallowing password authentication and good security measures. If the device you’re connecting from isn’t something you’re bringing with you, you’d also need a copy of your private key.

Almost any distro will work for pretty much all of that. Only potentially limiting thing I see there I don’t think that the Mullvad VPN client is generally available within distro repositories. Their site says it has clients available for Ubuntu, Debian, and Fedora. So perhaps pick from one of those?

For the steam link question, I know if I have a game running on my desktop it presents me the option to stream to my laptop (both Linux machines). I assume this is the same functionality. I don’t use it so can’t speak to how well it works.

Edit: It occurs to me this comment lacks a recommendation. Within the three listed supported distros for Mullvad, I think I’d recommend Fedora for you. Been some years since I used it last but I remember the installer being fairly robust and intuitive while still easy to use, and a pretty rock solid system once it’s up and running.

OpenBSD on an old office PC I picked up from University surplus.

TL;DR: I don’t daily drive (Free)BSD because it doesn’t work with some hardware and software that I use.

I’m not an expert in the BSD’s. But I’ve tried to daily drive FreeBSD a couple of times and can speak to it for myself from that viewpoint. Most recent attempt was about a year ago.

The issues I had:

Poor WiFi support. I understand that wireless driver support is iffy and even if it does work, it has poor speeds. I tried this on my laptop and could not get WiFi to work. There was a workaround in passing wireless card through to an alpine Linux guest VM and connecting there then creating a bridge. I did get this to work, but I found it impractical.

Games (via Wine/Proton) aren’t really an option. I understand you can install steam via a Linux compatibility layer and it’s somewhat functional with workarounds. I did not try this.

Those were dealbreakers for me. But the desktop experience itself was good. I ran KDE, and it felt the same to me. if I was running it on a wired machine that wasn’t used for gaming, I think I’d be happy to use it.

You don’t need an NVR. The camera supports a microSD card to store footage. It also supports FTP upload.

HomeAssistant isn’t required. It is supported and I do have it configured.

I have next to no recollection of how UniFi works so I can’t speak to that. Mine is connected to a 2.4GHz WiFi network. IIRC that’s all it supports. Might be different with newer models.

It also supports HTTP(S)/RTMP/RTSP/ONVIF streaming. Though I tend to just use the app (and I’m particular about installing apps) as I find it quite usable.

I bought a Ubiquity router once because a friend was raving about it. It crapped out on me after maybe 6 months? Will never buy their hardware again.

I’m using a Reolink doorbell camera and am happy with it so far. Had it for about two years.

I have a PC that I use as a router. It’s power setup is inverter > lifepo4 battery > battery charger. Relevance here is that it’s a “dumb” setup insofar as the PC can’t manage the battery.

What I did to help with battery health is figured out how long the battery should run the PC and have a scheduled disconnect of the mains power for about half that time once a day. Not as good a solution as something that can maintain a ~80% charge, but better than leaving it at 100%

It’d work for an old laptop too, and can be accomplished with a cheap aquarium light timer.