try the maurvin B , mlf and colds stabilising , I assure you on a cold climate grape wine, it will produce the best results.
the malic will be metabolised by the yeast softening the wine , mlf will enhance complexity convert the remaining malic to softer lactic acid and bring out the fruit and if needed cold stabilisation will drop some tartaric further softening the wine. if this approach can work on frontinac , it can work for most high acid wines.
carbonate offers none of these benefits , and involves adding something thnot normaly a part of grape winemaking practice.
if you are working with 16 g/l type fruit , you could do the fermentation and stabilisation protocol above and if after all that and cold stabilising its still to sharp , I would reccomend blending in some low acid wine instead of carbonate .
some of the best cold climate wines I've tasted have had some central valley or lodi wine blended in atg barreling . some cv cabernet sauvigion is a great way to add a little tannin (something many hybrids lack) and since alot of it is about 4.5 g/l TA it can balce a high acid wine.
10-20% of this blending can work wonders
if you must use carbonate most of the literature says to add your carbonate prior to primary fermentation minimising any potential negative fruit impact.