My first born was a girl and loves to latch. So I introduce bottle fed when needed. Why? FM makes them full as compared to BM eventhough they latch pretty long. It can also be that they have stomach upset hence the whine and grunting. And at this modern era, we have brands that has bottle teats that replicates mum's nipple. The only reason they loves to have BM direct latch because of mum's smell, bonding that's all. Baby wont be confused between bottle teats and mum's nipple. They will get use to it. If you don't start to introduce bottle feeding whenever needed, you will have difficulty to wean off when the time comes and it will take months, years.
Having said that your nipple need rest too, you can't let them latch too long or your nipple will be blister.
You can also do express milk meaning to pump and bottle fed, you may perhaps see changes in the feeding period.
Alternatively, last option to have soothe (pacifier) to let them suck on it when needed.
Hope this small tips help.
Sherrie