How complex is making a roll-your-own NAS?

It really depends on what you want out of it. I personally installed ProxMox on an old gaming machine (DDR3 RAM old lol) and have an Open Media Vault virtual machine running on it with access to my ZFS mirrored pair of storage drives.

Enabling Samba support in Open Media Vault gives you a nice little NAS. I believe it's okay to install bare metal if you really want to also.

It also has a nice Docker interface, so although I should probably not bundle services together so tightly, it runs things like Jellyfin for media, Paperless NGX for document storage, and NextCloud AIO for a convenient (if slightly resource-hungry) interface.

ProxMox lets me do fun things though, like back up the VMs, spin up virtual machines for PiHole ad blocking and Klipper for controlling my 3D printer.

My most important data gets synced to a subscription to a service called iDrive as my offsite. Pretty affordable for 5TB and my own encryption keys. :)

I want to stress that I'm not an IT professional or anything either. If you're reasonably comfortable with Linux and understand some basic networking, I'd say at least getting Proxmox and/or Open Media Vault up and running so you can access it on your home network isn't too hard.

Outside of that, and if you want HTTPS and stuff? There's lots of guides but I would recommend using TailScale instead of opening any ports to the web.

Sorry if this post was meandering but hope it gave you a little bit to go on! :)

Really depends on what you want out of the system, what you can spend and how much time you want to spend on it.

My old z390 itx system has a 16x PCIE to 4x m.2 card - leveraging an m.2 to 5x SATA adaptor with the built in SATA adaptors has given it plenty of space.

Considering I can grab m.2 to 6 SATA adaptors and fill the remainder of the slots that's a decent chunk of drives from a single PCIE x16 slot.

Software is another kettle of fish and a good way to timesink, I'd rather not give too much of my personal experience as there are so many ways to skin that cat.

It’s not too complicated but you don’t get some things for free like with Synology. It require work to setup scripts for offsite backup for example whereas Synology has a backup app with a UI.

For storage, I used to run ZFS in a raidZ2 configuration. If you do this then I suggest having a cron job running a script that can alert you if the pool is unhealthy. This is again something that Synology does for free.

You could also look up trueNAS core and see if that’s something that fits for you.

I have mini-ITX board in a mini case. 4 bays, 16 GB RAM of DDR3-L and a slow but very low TDP CPU. This thing is very low power but it's on 24/7.

Runs home assistant with zigbee, rtl433 and whatever it detects over the network. A few older game servers (minecraft, minetest/luanti, quake 2), miniDLNA, ... Arch Linux, so rolling release and always up to date with the latest versions.

Served me greatly and I haven't upgraded because it still does what I want and I can't find any modern CPU with a TDP this low.