Meh, imo spoilers only spoil experiences for people who take media too seriously.
When I consider watching a movie, one of the first things I do is read a complete plot summary, including the ending. When I do this and no longer want to watch the movie, in my mind, that’s not a sign that any experience was spoiled, but rather that it just wasn’t very interesting to begin with.
Conversely, I have played Nethack on and off for decades, have read countless spoilers about it, yet still haven’t won and still find it interesting.
I disagree with your movie watching routine. A movie is more than just its plot.
I play a lot of experimental games where not knowing what the plot is is the point of the game. Doki Doki Literature Club is one such game. The experience is the point, not the plot.
Many films are meant to be experienced, not just read or watched. Otherwise what's the point of a movie when you can just read a screen play? Or what's the point of a screen play when you can just read a synopsis?