> The wild card: M1 Mac mini

From $899 + external storage

Creating your own NAS using another Mac is straightforward. Connect the Mac to wired ethernet, then connect external storage you’re going to use for shared backups and storage. Format that storage in APFS, encrypted if you wish, using Disk Utility, giving it at least two volumes — one for Time Machine backups, the other for files.

Open the Sharing pane, select File Sharing, and add both those volumes to the list of Shared Folders with appropriate users. Turn the backup volume into a Time Machine destination by selecting it in the list and

From apple.com

clicking Options…. In the Options dialog, tick “Share files and folders using SMB”, then Control–click the volume and select the Advanced Options… item. In that dialog tick “Share as a Time Machine backup destination”. If File Sharing isn’t yet enabled in the list of services at the left, tick it to turn it on.

Finally, open the Energy Saver pane, and ensure the Mac is set not to sleep, so that it is always available for backups.