Best client for macOS / iOS?

[simoncn]

The UPnP protocol doesn't provide a way for a server to send images for artist, composer, etc. The server can supply images for the selections in the Artist browsing container (similar for Composer, etc.) and all good UPnP control points would display these images when viewing this container.

The UPnP protocol also doesn't provide a standard way for a server to send URLs for release information, etc. MinimServer is doing this in a nonstandard way for PDF booklets but very few control points are able to handle this.

This means the limitations you have described are a result of limitations of the UPnP protocol rather than shortcomings of UPnP control points. This protocol was designed in 2001 and would greatly benefit from a revision to enable additional metadata to be sent in a standard way.