One interesting difference with our company is that I hit the bottleneck of "I need to be doing more spec work and less engineering work" more quickly - possibly because we were relying on contractors who couldn't (or wouldn't) take on as much of that spec work themselves. I started to really feel this pain point at 3-4 other full-time engineers, where I was sort of trying to stay ahead of them and make sure that upcoming features were fully specc'ed and met the business needs as opposed to chipping in and coding myself.
I'm definitely tinted by my own experience, but it seems like 8-10 other engineers is a really late point to hit this bottleneck - did you have a PM that was also working to help plan the features to reduce this burden on you?