I’ve always used a duvet for my little boy and he loves it. If he gets hot he normally kicks the blanket off. I’ve never had a issue with him using a blanket x
We’ve been doing duvet on milder nights bc he’s a wriggler too. And sleeping bag when it’s cooler. It’s working well so far x
For my son we found no difference in his actual sleep between duvet and bag. However when we changed to duvet he woke us up a lot less when he wanted to come in and cuddle with us (he kept tripping over in the sack), so we all generally got a better night's sleep on the nights when he does sleep.