a simple cron job pointing to an update.sh with an apt update && apt upgrade -y does the trick.
i wouldnt recommend you to completely automate it though
debian has unattended-updates by default and generally takes care of itself
in addition to servarr, which was already mentioned, you can use Stremio. its similar to popcorntime and a lot simpler than setting up and maintaining servarr.
an RPi’s processing power can handle your use case as long as packages are available for its architecture, it shouldnt be a problem on common usecases.
you can use it to stream games from another machine using sunshine/moonlight too.
only if it goes through their servers