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

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

I have used Audirvana on a MacMini in the past and found it reliable and simple to use. One suggestion that keeps things simple is to put your music files on a separate external SSD connected to MacMini via USB or preferably Thunderbolt, the latter being faster. The rationale, which is used on Roon servers is to keep the OS and Roon core software on one of the server’s internal drives and the music on a separate internal or external drive. The reason is reportedly to speed up operations by not having the same drive hosting both, in your case the Audirvana software and the music files. I received the same instructions from Audirvana so you might check with their current support folks. 
 

This approach eliminates the need for a NAS and keeps things simple but fast, using only Mac OS and Audirvana, Also, I added a two hard drive RAID connected by Thunderbolt to back up my media files. 

Melco N100.  Never an issue in greater than 5 years.