Tricky to say. 20-30GB isn't unreasonably small for shards so IMO you could just leave them at 12; OTOH there is usually some data duplicated across shards (e.g. the terms dictionary) so shrinking might save you some more space. You don't have to force-merge anything, and again it's tricky to say whether it'll improve things further. I don't think you'll find another 40% of space savings but it depends on the details of your data.
Sorry there's no definitive answer here, I don't think we can offer more guidance than to try it and see. You've certainly got space to experiment now 