Agreed, that's the route they should take in RDR3- have it set in the start-to-height of the Old West, and make it a narrative/gameplay sandbox with various plotlines/goals and endings depending on your morality and actions (i.e. the spaghetti western good guys/bad guys).
Stop having all the missions be on-rails setpieces, and instead create more open-ended missions that give players the ability to accomplish goals in a unstructured manner. The story is left untethered to RDR1/2, and your character becomes part of the legends of the Old West regardless of their actions.
RDR2 is completely railroaded, you can even fail missions by doing the wrong thing, or going the wrong way. I agree that they missed out on multiple ways to accomplish missions, and even discover endings, there should have been options to allow Arthur to leave the gang, or survive (i.e. escape to New Austin) if he made certain choices.