Looking to build something like synology

I currently have a Dell power edge r630 with 256 gigs of ram and eight 500 gig sas drives.

I’d like to be able to make something smaller like a synology or qnap but have the ability to maybe plug in a DAS for more storage if needed. I have a 1 year old so im on a tight budget of 300 and at MAX 500 dollars. Im struggling going through the build guide as there is sooo much to consider lol.

I use it for plex, jellyfin, audio library, Vaultwarden, etc (unraid).

Currently im using windows server 2022 though I think im going to move to unraid as it seems more simple and will deal with containers through that.

Help please lol