Ask HN: Designer to FrontEnd Developer Handoff Process?

Background: We are primarily a mobile web company. Our current process is our designer mocks up the screens in Sketch. One the designer and product team confirm a design, the designer outputs a PNG that is the height/width of an iPhone 7. That height/width then goes into the requirements document.

Problem: There is a lot of back and forth between the front end developer and designer about spacing/font sizes/general look and feel as the mockup turns into a responsive design. I am hesitant to have our designer mock up 4-6 breakpoints of how each screen should look like, but maybe there is no way to get around it.

Question: I am wondering what process y'all use to minimize the back and forth between the front end developer and designer? (any tools, systems, etc that help)