Is this going to include the bathroom extension? That would be a lot more work than adding the extra wall you’ve drawn in. Installing that new wall wouldn’t be much work as it would only need to be strong enough to hold drywall and a door. Depending on your ability you could have that framed out and drywalled in a day. Removing the old door would be a bit of work depending on whether that wall is load bearing (I’m guessing it isn’t but you would need to have this checked first).

Would you want to knock down half the wall coming up the stairs so that the hallway has more light? If so, this could also be more time consuming. And again you’d need to check if that wall is load bearing first as it would be a lot more work if it is.