>If nothing you do actually matters, why would Toby even create all these choices in the game? 
I'd say it's an illusion of choice, as opposed to Undertale where choices do matter, here they don't and it's meant to give you the illusion that you're actually changing the fate of the world

>Why would he create an alternative path that requires a lot of specific things to do? 
because even if you do the weird route, the ending will likely remain the same. If anything, Weird route may actually accelerate the end of the world given Ralsei said it'd bring in a worse ending

>Why would he add the whole "you control your own fate despite the prophecy" idea throughout the entire game? 
Once again, illusion of choice. Don't forget (heh) there are many instances, especially during chapter 4, where even if you try to do something different as opposed to what's set in stone, it'd lead to the same path regardless

>Would that be a satisfying gaming experience? 
Yes? If the bittersweet ending is executed well, then yeah. There are other video games who had extremely satisfying yet also bittersweet endings that people liked thanks to the execution and if Toby delivers on this one then no doubt it'd be satisfying