Sounds like you are an overachiever. The big implicit assumption of GTD is that, while it makes you more productive, there is still a limit on how much you can take on. Workload is not specifically addressed, but implicitly the overflow will go to the Someday folder.
I think the answer to this problem is not any productivity trick, but to aim for exponential growth of your business, career, etc. so that Delegate becomes the primary workhorse instead of Next Actions. Prioritize actions you perform and projects you start so that they are conducive to a long term self-sustaining increase in your productivity. And accept that you may have limited success/luck in unleashing the exponential and that some projects will forever stay out of your linear reach.
I think the answer to this problem is not any productivity trick, but to aim for exponential growth of your business, career, etc. so that Delegate becomes the primary workhorse instead of Next Actions. Prioritize actions you perform and projects you start so that they are conducive to a long term self-sustaining increase in your productivity. And accept that you may have limited success/luck in unleashing the exponential and that some projects will forever stay out of your linear reach.