Yes I have, and it would end up costing more than the WiiM, and any app would not be better than the WiiM one which gives you Spotify and Tidal Connect out of the box, and support for most other music services too.
You mention you have several other devices connected to your router, well have you tried connecting another established brand streamer? Maybe it's not the WiiM specifically, just the nature of being a streamer.
Yes. I have Chromecast Audio. I also have Chromecast and FireTV devices.
The software is Netgear Armor which is basically Bitdefender rebadged. The software sits at the network perimeter rather than on individual devices, though it can be installed on PCs, phones etc as well.