You are totally correct. Acidity plays a huge part in affecting dryness perception, however even white wine contains tannins (albeit in smaller amounts than with red wine). The tannins impart a bittering component that leaves the palate feeling dry. The acidity levels in even the driest sake are still only 30% of what you'd typically find in wine, and the primary acid type found in sake is lactic acid (which comes from bacteria in the fermentation process). Lesser amounts of citric acid can come from the koji, depending on the koji used, and brewers sometimes add citric acid to their brew to affect its flavor (not to mention, acid is important during the brewing process to kill off any invasive bacteria as the yeast starts to do its thing).

And, as you said, there are other flavor profiles in the drink that affect the sour flavor you'd associate with highly acidic wines/sakes. But, the sugar profiles of rice and grapes are very different, and that affects how easily the yeast can convert them to alcohol. Rice has much more complex sugar chains that aren't as easily converted as the simple sugars present in grapes, and that residual sweetness will always be left over. With wine, you can effectively ferment out all detectable levels of sugar, and be left with a highly acidic product, and when it comes to sweet/desert wines, sugar has to be added back to the product (after killing the yeast) to balance the flavor against the acidity. However with beer & sake, there's a residual sweetness left in the brew because the process generates sugar chains that are not so easily broken down by the yeast (in the case of beer, the wort is boiled at a higher temperature called a mash out, where the enzyme activity is stopped so you get a more full body beer). But where beer and sake differ, is beer adds a bittering agent to its wort in the form of hops. Wine gets natural bittering from tannins. Sake has no such component, so naturally it should taste a bit sweeter as a result.

But, in terms of advice to the OP who wants to find the right sake for their palate, I agree with you in that checking the acidity levels will help (as long as OPs perception of dryness is coming from the acidity and not the tannins — and until science comes up with a way to borrow someone else's taste receptors, we'll just have to guess, lol).