I have a Chromecast that I often use with the Chromecast extension to stream movies straight off of my NAS, but when I'm sitting on my couch, I can't get to my computer to click on the pause button. Is there a way (or an extension) that I can interact with it using a wireless keyboard? Perhaps an app for my iPhone?
Yes, a program I use called plex. https://plex.tv/ I believe they offer support for NAS devices. They have an app for iphone, android, and windows and I use it on my chromecast. Your solution may require a small fee, like $5, but it was worth it. There may be other programs like it if you search for it.