Pour-over coffee tasting hollow when grind changes seem to fix bitterness but not sweetness

I'm getting an oddly empty-tasting V60. Going finer adds a dry, bitter finish; going coarser removes that bitterness but makes the cup watery rather than sweet. I'm using 18 g coffee to 300 g water, roughly three minutes total, and a medium roast that tasted good at the shop. Water is just filtered tap. Before I keep moving the grind setting back and forth, would you experiment with a tighter ratio, hotter water, or a gentler pour?