First NAS Setup what to buy and install

So, first of all massive thanks for all the help on my previous post [https://www.reddit.com/r/HomeNAS/s/Uaf1QslFMQ](https://www.reddit.com/r/HomeNAS/s/Uaf1QslFMQ)

I now have a UGREEN 4800GT but nothing to put in it yet.

What I want to do with my NAS (that I know of) -

* Primary Use - Backup photos videos from me and immediate family. Currently have maybe 2TB of them but now have 4k drone and action camera and getting out more so will greatly increase. I want to get off Google/Amazon etc. and focus on privacy. * I will backup the NAS to a as yet purchased external drive that I will keep away from the NAS (fire/theft). * Possibly run my Reolink cameras on it as an NVR. Currently they only use the SD card. I'm thinking maybe to come off the Reolink software to lock it down etc. * All desktop and laptops are currently on Windows but I'm seriously thinking about putting Linux on where I can for privacy. All Phones are Android for now. * Router is an Eero 7 with fibre at 2Gbps up and down. Connected to a UGREEN 2.5Gbps switch. My desktop is wired to the switch but is only 1Gbps (new PC maybe soon). * Making the odd YT video, edit from the NAS? * Maybe Pi Hole and VPN

So, what am I asking?

Your recommendations on what to install/buy?

4 bays plus NVMe space, do I get an NVMe to run something on?

Do I only fill maybe 2 or 3 HDD bays and leave one space as I don't yet have much data? can I easily do this?

Which OS would you go with, maybe not use the UGOS incase of privacy issues, TrueNAS maybe?

Upgrade the switch?

TIA, will add if I think of more stuff; which I probably will as my memory is kapput!