Welcome to the forum. Pretty sweater!
You can try going down a needle size on your swatch but it may be easier to convert number of rows to a length measurement. Use the row gauge of 32 rows per 4 inches (or 10cm) to figure out the length to work.
You could also adjust the placement of the increases and decreases if necessary. Rather than working the given number of rows, convert that number to inches and then space out the increases or decreases.
First try going down a needle size as you suggested and see how much that changes the stitch and row gauge.
There are several projects where the knitters compromised on stitch and row gauge in order to get the size that they needed. And again as you suggested, you can get row gauge then figure out which size to make given the new stitch gauge.
In addiion to your plans of trying a different size needle or recalculating the size, you might try rewashing the swatch and hanging a weight on it whilst it dries. I have not done this with Drops Belle but some people recommend weighting a swatch to act like the weight of the yarn of the full top. I have had sweaters extend more than I had expected due to the weight of the yarn. It could be that it draws the stitches down making fewer rows per 10cm than it does without weight and flat.
I’ve used this yarn twice recently and one came out quite big and the other a bit small, all swatched etc but one had more lace and grew more than expected.
If you have the motivation for it you could make a large swatch or if the piece is made in pieces make a back up to armholes and block that to see a closer idea of gauge.