I definitely think you should wait for the fermentation to complete and then rack and add sulfur dioxide. I doubt you’ll have to add any sorbate to protect against a re-start fermentation because I believe your wine is almost dry! Every wine is unique and as such hitting the “perfect” dry number is almost never possible. I’ve known dry wines to be completely done at 1.000 specific gravity (SG) (0 °Brix). There’s no magic in absolutely needing a wine to cross a 0.996 or even 0.995 (-1 or -1.3 °Brix) line. Some wines may have a certain degree of unfermentable sugars in them, believe it or not. I don’t have any specific (no pun intended) experience with elderberry wine but it’s possible you’ve got a must with an unusually high amount of pentoses (five-carbon sugars), which yeast can’t ferment. I’ve seen plenty of commercial wines in my career be “done” fermenting, taste dry and have 3 g/L “residual sugar,” and my usual “OK, it’s dry” target is 2 g/L. In fact, I don’t think I’ve ever seen a 0.00 g/L residual sugar wine in my entire life.
I would check, however, that your SG hydrometer readings are accurate. Hydrometers are an instrument for reading density of liquids and are usually calibrated against water at 60 or 68 °F (15.5 or 20 °C). The first thing you can do is check the calibration and ensure it is properly calibrated. If it is off in water at the recommended temperature, be sure to incorporate how far off it is into any future measurements. If your wine sample is warm, your hydrometer will sink, giving an artificially “dry” reading. Conversely, if your wine sample is cold, the hydrometer will float too high, giving you an abnormally “sweet” reading. Sometimes tiny bubbles from a fermentation can stick to the glass or plastic sheathing of the hydrometer and act like mini life preservers, causing the hydrometer to float artificially high. In your case, with what seems like an active, bubbly fermentation, I’d be willing to bet that some clingy little bubbles are adding slightly to the “float” you might be experiencing. Also be sure you’re reading your hydrometer lines at eye level.
One of the big tells when a fermentation is complete is when it stops bubbling. Is the hydrometer reading still actively going down or is it stopping/settling at 0.998 (-0.5 °Brix)? If it’s still moving slightly every day or so don’t worry about it, you’ve still got an active fermentation. Does it still taste sweet? If so, and if your SG readings are moving and you’ve got plenty of bubbles like you report, you likely still have an active fermentation that just needs to finish and take its sweet (ha, I did it again!) time. Have you checked it with a Clinitest tablet to see if that registers any residual sugar? If a Clinitest assay registers dry, your hydrometer has stopped going down and you’re still having lots of bubbles, then it’s likely you’ve got a secondary fermentation happening, which could signal a spoilage organism. If you think it’s dry, rack and sulfur it.
