Is it possible to use a single PC for windows gaming and then boot switch between that and RAID NAS storage
I have a PC with
Ryzen 5500
16GB RAM
RX 6650 XT graphics card
B550 gen 3 msi motherboard (with 6 sata ports)
120GB SSD (currently what it boots windows off of)
250GB SSD
2TB HDD
8TB HDD
Is it possible to grab a 64GB usb stick, throw some kind of NAS OS onto it (I've heard of unraid, truenas), create a setup where if the USB stick is plugged in the PC will boot to NAS OS and ideally using 3x 8TB HDDs in RAID 5. If USB stick is not plugged in, then booting as normal into windows and use the 120GB SSD, 250GB SSD, and 2TB HDD for gaming purposes?
I'm aware booting off USB stick may suck, so I'll probably end up using one of the SSDs as the NAS OS boot drive instead - just curious how viable it is to use a single pc as two different computers
Yes it is possible and absolutely viable as a starting point. You might will start experience some drawbacks of hosting on a Windows desktop system more sooner than later. Reboots due to system updates as example.
Use it, and think about getting an use office PC that can hold your drives.
I think that should be possible but I would prefer the second option mentioned - Hyper-V role with a NAS OS Vm and drives passed through to it. Then collect in RAID inside a VM.
It's absolutely possible. My recomendations: Do not boot an OS from a USB stick, USB sticks are made with recycled donkeycum and will quickly fail when reading/writing as much as an OS neeeds to. I'm sure there is a boot loader that can help you choose wether to boot into windows or your NAS OS everytime you reboot.