Best client for macOS / iOS?

[simoncn]

The main advantage of OpenHome is that the playing queue is held in the renderer. In standard UPnP, the playing queue is held in the control point.

Other differences you are seeing are caused by the implementation of the Linn control point you are using (presumably Kazoo). When Kazoo is used with MinimServer, it is subject to the limitations of the UPnP protocol. When Kazoo is used with Kazoo Server, a proprietary Linn protocol is used instead.