Overwhelmed

I'm trying to lurk my way into building a new and improved home server. I'm currently running a Raspberry pi5 with a 126gb micro sd and 1tb external attached to it. Running Jellyfin on PiOS using Tailscale to access remotely when I'm away from home.

My goal is to give access to 5 friends or so (not really simultaneously cause their free time schedules usually don't overlap) to watch movies from my collection. All I want to store and access are films and shows. But I plan to build a larger collection, surpassing the 1tb external I currently have, with a budget of around 500$.

I'm unsure, do I get a NAS?

Is it better to find some office computer and repurpose it?

Unraid or not?

Docker or not?

I know everything is subjective but what would y'all recommend.

I also get lost when it comes to the security behind my server. I know very little about protecting my assets and again like most things there are many options, most of which I'm not familiar. If you were new how would you familiarize yourselves? Are there some checkmarks I should hit? Is there a guide I should read/watch? I truly don't want to just be spoonfed I'd like to get active and understand what goes on underneath the hood.

If there are some prereqs I should've mentioned in my post let me know and ill oblige.