Beer is all about balance which you've already discovered. If you are buying kits then buy kits that are giving you what you want as they are already designed to be balanced. If you want to tinker with a lower alcohol kit then you need to add mor sugars in the form of LME/DME but then you also need to add hops to balance out the sweetness you've introduced.
Pitching more yeast does not raise the ABV
