How to build a watchlist you actually finish
A watchlist is not a wish list. Four hundred titles is a place things go to be forgotten; twenty is a list you finish.
Cap it, and mean it
Keep the list at a size you could get through in a season — twenty or so. Adding something when it is full means taking something out, and that choice, made once a week, is what separates a queue from an archive.
Add from lists, not from feeds
A ranked list of the best films in a genre gives you titles somebody stands behind. A social feed gives you whatever was released this week. The first is a shorter path to something you will be glad you watched.
Mix the lengths
A three-hour film and a sixty-episode series cannot compete for the same evening. A workable list holds both, so that "nothing to watch" is never really about time — there is always something the right length in it.
Drop things without ceremony
Two episodes in and bored is a complete answer. Mark it dropped and move on: a list you are allowed to abandon things from is a list you keep using, and a dropped title is still data about what you like.