That's awesome that you're launching a YT channel! It sounds like a cool concept, I hope it really takes off! 💚
I'm not positive I fully understand what you intend to do (and thanks so much for asking!) but it seems like you'll be shifting bits of the story, concluding other characters' roles, and creating an ending?
Honestly, that sounds more like a remix story. I'd totally support you just creating a story that's heavily influenced/inspired by this RP, and credit all the great players who made it what it is. By altering what's actually posted, you're creating closely-canon fanfic to fill in the gaps that you found, that is also clearly your own speculation and not how the story actually happened.
It's a subtle difference, but one that I think is important, respectful, and would be a compliment to everyone who contributed and cared about this story. On a personal level, I'm not comfortable with someone else writing an official ending to this RP or other players' characters, but if you're presenting it as a fan work, absolutely go for it.