I tried selecting Reaper route at the point where you supposedly can select the Poly option but it seems that Reaper route isn’t available once you start flirting with Trapper, it seems.
Viewing post in Devlog - 11-11-2023 Roadmap & Walkthrough comments
You have to die enough times and get all dialogues with Reaper so he gets included and you get the poly route with him and Trapper :,D
I realized that later after looking back at the walkthroughs and noticed how much we have to die so we can actually pursue Reaper 😅
You'll notice when you have to stop dying in an area if the next dialogues with Reaper/death scene do not change anymore (probably. idk, I'm guessing for now since the Waterfall walkthrough isn't done :,) )