I use a tool called Series Guide (https://play.google.com/store/apps/details?id=com.battlelancer.seriesguide) to track the various series i am already watching/interested in. But for movies generally we add the to the list and then choose from it if we fancy a film. But I suspect as it's usually just the two of us that's an easier decision.

Even though I pay for a handful of streaming services I'm subscribed via RSS to a pirate scene releases log. If something catches my eye, which doesn't really happen all that often, I add it to Radarr which downloads it to my home server / Plex. Other times me or my girlfriend add movies to Radarr due to word of mouth or some other reason.

This library is basically stuff we have an interest in watching so when we're up for watching something it doesn't take long to choose.