I really would advise against a raspberry. Even a Pi 4 is too slow for transcoding and storage over usb is just too unreliable. I would go with a motherboard with integrated CPU. There is a really good one from Topton with a Celeron N5105 that supports hardware transcoding. I did a build with that recently and I’m really happy with it so far. Power consumption is around 35W with two 18TB sata drives. If you only use one and have it go to standby when not in use you could go even lower.
If you want something prebuilt there is a NAS from terramaster with the same cpu. It uses a an internal USB for OS storage that’s easily replaced. It’s called F2-423.
I did something similar. I used Proxmox and then installed PfSense as a virtual machine. You might want to look into OPNsense and PfSense. There are addons for both that do the same as AdGuard and are free and open source. I had good experiences with pfBlocker