Yes I was thinking that.would be worth speaking to a lactation consultant if you can. In the meantime it would be worth trying breast compressions when feeding to encourage flow and get LO busier, that might help reduce the time. sorry can't link but google Jack Newman breast compression and you'll get a video to see how it's done.
Glad to hear you don't feel you need to top up every feed
lots of mothers successfully mixed feed but it's important to get the balance right so you protect your supply.