Any simple, reliable stand-alone (non-innuos, Sonos) NAS products out there?

I have not found the Zenith mk3  a problem currently  IHave had no issues  updates 

they let you know as long as you have it on , very smooth  . What. Has  been your issues ?

Why a nas? I spent the last 20 years of my career in the high tech storage sector, worked for the largest hard drive/solid storage manufacturer in the world.

I use a newer M2 Mac mini as my server and it acts as a nas for all my other computers/devices. This Mac mini has a very large disk drive for backups from all my he other Mac’s/iphones/ipads. I also have 10TB of ssd/nvme m.2 storage to share amongst all my other computers/devices.

I also have a Linux (Mint) server running Roon and I share the Roon music files off the Mac mini server. I have a 128g ssd on the Linux server for the Roon backups.

You do have RAID options on the Mac for NAS (0,1). If I wanted to use RAID 5 or RAID 10, I would buy a 5 bay external ssd storage device with included RAID software and hook it up to the Mac mini. This Mac does a lot more than just serve up shared storage, it handles all my home automations and other programs with power to spare. I ran Roon on this server for a while but moved Roon to a Linux server that sounds better. 
 

@ml8764ag you should look into Synology NAS products.  They are commercial grade but have a nice easy interface.  I have used one for years (a two bay) for my file storage and streaming in-house.

Synology makes an application called DS Audio that talks directly to the NAS and you can scroll and play music that way, but I also use a program called MusicStreamer on my phone to find and play music, but can direct it then to my fiull sized stereo if I desire.

I also can use JRiver and Roon (which you didn't want) to play to my stereo and those programs find the files on the NAS.  My Yamaha receiver also has an interface that lets me browse shared folders on the network and then play directly to the stereo.  There are many choices.

Another option is a Raspberry Pi 4 or 5.  You can add Lyrion Music Server (LMS) to the Raspberry Pi OS (formerly known as Raspbian) or, if you never want to use the unit for anything other than as a music server, install piCorePlayer software.  Then add an external USB drive to the unit with your music files. You now have a music server. The only catch would be if your player(s) don't offer Squeezelite. But even then, LMS has a UPnP/DLNA Media Interface plug-in that allows it to be used that way.  There is a bit of a learning curve to setup a RPi system, but once done it is a solid performer. 

I think the more general-purpose suggestion of a Raspberry Pi is going against the desire for this to be simple.

The OP wants an NAS, which isn't unreasonable, as they can simplify the data storage and backup aspects of the process.  It would help to have more information such as the type of networking available (wired vs wireless, for instance), the amount of data to be stored, and the need (or not) for other data needs like video and non-media data storage.

Synology is a good option in general.  Asustor might have the better hardware, and QNAP makes a wide range of products from small and simple to ones that scale significantly.  Terramaster has some good offerings, but I think their hardware and software is not quite as solid as the other.  The relative newcomer is UGREEN, and I don't know much about their products, although they seem to be priced reasonably and have generated some buzz recently.

There are several choices here, and with more information on the use case we might be able to pare down the recommendations further.

Michael