It's not exactly one wrong choice, its all your choices in total. Thus, you will not be able to replay from some certain point. You will have to play from the beginning and try a little bit harder to deserve his true love and these sweet scenes.
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This makes me a little sad, because I tried to follow the best choices on the route, maybe a few that I didn't think influenced the story so much, those were really my opinion, I always tried to give Spencer a lot of support with what he liked. But after playing the epilogue and thinking a little, I think maybe it has something to do with the choices related to Darius, but I'm going to try to do some tests to find out, do you think maybe I'm right?
I didn't inspect Spenser route choices so deep. I am also didn't get the best ending for him in a first try. Maybe someday i will go through his route code, but right now i didn't have certain advice for you except just play the game and engoy your time with him. Or try some other routers and comeback to him with fresh look on every decision. But if you want his best ending so much - visit Dyne's patreon. There are guides for best endings at some level of support.