I like you theory a lot.
Seriously Vader is the chosen one and he does bring balance to the force in Ep.6 when he kills the Emperor essentially ending the Sith in one swoop (he obviously dies as the last Sith). To me Jedi was well written. You can tell several times during the movie that Vader is slowly drifting back to the light side.
He refers to Luke as “my son” when he says it’s too late for him.
He senses Luke on the stolen Shuttle yet the Emperor does not, clearly he sensed him through the light side of the force.
And eventually his love for his son brings him back completely to make the ultimate sacrifice to kill the Emperor (even with the cheesy “NO” added later).
That said you theory is still well thought out but I don’t see how any one can say this character or that character brought “balance” to the force. Technically balance is when both sides are equal. If anything Anakin/Vader brought imbalance to the force when he killed himself and the Emperor tipping all the power to the light side and the only standing person in Luke. If anything Ben Solo turning to the Dark side brought balance back.
I’m more curious who Snoke is. Many people think it could be Darth Sideous/Emperor’s old master Darth Plagueis. In a lot of Star Wars expanded universe series Darth Plagueis was working on immortality through the Dark side when Sideous murdered him. Who knows but it would fit.