You could try sur-lee aging of your cider, though you have most likely racked already.
The yeast cells breaking down will release mannoproteisns giving mouthfeel. You will also pick up some other flavors that will give complexity.
If you have already racked, products such as Opti-white or Noblesse from ScottLab will accomplish the same thing.
Next time you could use a glycerol producing yeast for a fuller mouthfeel. 71b is one (though it can produce some aromas some don’t like - with some apples it makes aromas of Grandma’s pot pourrii). There are other glycerol producing strains.
I’m not aware of any tannin additive products that work well with cider.
Despite being almost categorically opposed to adding non-pomme fruit to cider, I have had some that were co-fermented with blackcurrants that had a pleasant tannin profile.
At TA 6, it’s probably a bit sharp as a dry cider. Adding <10g/L sugar will give mouthfeel, balance the acid and bring out fruitiness without being perceived as sweet. You will need to pasteurize the cider now. Or add sugar at serving. Blackcurrant juice could be added now to sweeten, but will be more glaring in the flavor profile.
Carbonation will also lift things on the tongue.
-'//es Cherry
Dragon's Head Cider
Vashon Island, Wa US