Let's say I'm brewing perfect v60 cups for my taste and don't want to change the flavor as its not too sour or bitter. However I just want it a bit weaker. What parameters would I change? Could I just add a little water after the brew? If I used less beans or more water during, I'd think it'd mess with the extraction.
Yep, adding water is a perfectly good solution! You can do it a little at a time until it tastes right and then make a note of however much water that was.
Bypass will reduce your extraction and hence efficiency, but that only matters in a commercial setting IMO. That being said, if you want to achieve the same thing without bypass at the end, probably what you'd end up doing is using a longer ratio (more water) and then possibly needing to tweak another variable such as grinding a bit coarser to re-balance the flavor.