the roku is freaking awesome. miracast is built into android so most phones and tables with it connect directly to the roku stick with nothing in between. i watch movies off my phone all the time on my roku with no extra anything. why oay for flex when this already works
but you will need something to make this possible. if the tv is not a smart one then you will need something else. a ton of dvd/blue ray players have amazon built in. as does roku, apple tv, amazon fire stick. chromecast simply makes your tv another monitor for your pc so you can see whatever your pc is showing on the tv. but in the end you need something to do this for you.
as a side note, win 10 has miracast built in as well and it picks up the roku stick as well making the simple connection you want as well. again the tv is simply another monitor to the pc with miracast going. this does take newer wifi hardware however and even a couple year old wifi card won't work as it is not in the standard at that time. a new ac adapter will work fine to be able to use miracast.
right now with sales going, the roku stick is $40 at many places. i prefer it over apple tv and amazon stuff. the others cram their own content to the front and it can be hard to find other services. roku is 3rd party so everything gets equal treatment.