Hey man, I ride pretty much the same conditions down here in FL quite a lot, and split time between the 860/770 about 80% of the time. 1075 when things are truly shit.
In ocean conditions with the primary cross onshore swell mixed with wind bumps, it can be tempting to think the only energy is on the onshore swells.
The trick IMO is to see the wind bumps that are moving almost straight downwind, these will get crossed by the stronger onshore swell and look like the disappear, but if you’re high on your mast and trust that it will re-appear, you can go right over the big swell with almost no effort and continue to follow the secondary bump. Once you can do this, I believe it gets a lot easier to maintain your speed in between the bigger more obvious bumps.
That being said the overall approach for mixed directions you describe, ride the more powerful onshore swell slightly in to gain the most speed, staying high here is important. As you’re speed comes up you can start to “fade” more offshore and kind of wrap around the face and ride more parallel to shore, almost like you are riding the face of a wave. At some point this will run out and the swell will pass you, that’s when you stay high and give some good pumps as @Matt describes well in no mans land, this is where I try to find those 2ndary wind bumps moving straight downwind. Basically stay with these as long as you can maintain speed, or another one of those onshore faster swell presents itself as an opportunity to get your speed back up.
In moments of pure panic, @mgudwin is spot on to cut harder than you think you need to so the energy gets to you quicker. I ride with him all the time so can vouch he know’s what he’s talking about even if we ride very differently haha.
Regarding suggestion of 770R, I definitely disagree based on the issue you are describing. If you struggle to maintain speed in between the ocean swells the smaller wings will only punish your technique. However, I do think once you have the technique down, a smaller wing can be easier, as long as you do everything right.
Hope this helps!
*Last thing about the R’s I think being high on the mast is super important with them, Its a totally different feel and ride if your on the bottom 25% of the mast, move those feet back and stay high!