A small tank under the sink would not help the shower or other bathroom, only that sink. A recirculation system at the furthest point would improve all things, by bringing (at least) warm water to the point where the other things branch off. I'd put a recirculation system in. Do an experiment, unless you know which sink is furthest from the supply line, to determine which vanity is the best location for the thing. I have a RedyTemp. I have it adjusted only about 1/3-the way on the thermostatic control. This gives me warm water at the sink, nearly instant hot at the shower (closer to the supply), and minimal warm in the cold line I use for return. Flushing the toilet purges all of the warm water from the cold line, and that isn't an issue elsewhere since the thing stops before it gets warm too far back to the WH. If I wanted the water hotter initially, I'd have more warm in the cold line I use as the return. That's one nice feature that differentiates the RedyTemp from the other systems, you can adjust how hot (warm) you want it. Then, I have it on a timer so it only runs when I'm normally home. You notice the difference on say a holiday. Depends on the timer you have how easy it is to override. Some people put it on an occupancy switch, but that would only work if you went into that bathroom, not generically around the house.