I would say, as my number one piece of advice: Design with a mobile-first approach!
I know, I know, this isn’t a great option for you right now. You’ve spent a lot of time designing in a 1200px wide view. But I can’t exactly recommend you start from scratch if I’m being realistic, and I can’t recommend you moving forward with the current layout and design, if I’m being honest.
It may be a different story from other folks around here, but for me, changing my design view to 640px with a 20px margin on the left and right of a 600px wide column group made the most sense for designing a mobile-first app. Every time I place an object on the canvas I have to decide it’s purpose and it’s mobile factors (such as when this element gets hidden, when the margins collapse, etc). All objects I place within the 600px group are 520px wide with a left and right margin of 40px, each set with a minimum width of 30-50% (depending on the use case). After I am at a comfortable point in development of a UI view I jump into the responsive mode where I select which groups of elements can “Wrap to previous line if page is stretched” and which elements stay at a fixed-width of 100% max.
I wish I had an easier answer for you. My team and I have been developing the same app (or rather same idea, multiple versions of similar apps) for 9 months now, and 2 months into designing a 1200px wide view we transitioned to a 640px wide view for sanity sake. We know our customers will be using mobile a ton, so it made the most sense to figure out a mobile-first process rather than a mobile-second band-aid.
I can share more details of our approach if you’d like. FYI: Google, as the world’s leader in realms like this, are splitting (or has split?) their search engine into two parts: desktop and mobile SERPs. To me this meant planning, designing, developing and marketing a mobile-based app makes the MOST sense going into 2017. It doesn’t matter how well you can collapse a 1200px view app into 320px — the strategies for the two screen sizes are so different in every way. I say transition to a smaller viewport today rather than waiting. You will inherently design a better mobile app if every piece of data that goes into it is thought of in a mobile-first approach.
IDK how I can phrase my thoughts any differently than above. Feel free to reach out if you have questions on going in the mobile-first direction, I’ve learned a ton that I can’t succinctly share in the forums.