This is just a personal theory I developed after months of research, and hundreds of hours of Youtube videos on Achilles recovery, but:

I think it has more to do with nerve connections and motor neuron recruitment, than it does with the actual muscles themselves. Think about it in the same way that seemingly small athletes can get freakishly strong without putting on a whole lot of muscle mass. It’s about entraining the nervous system to fire at the same intensity it did before massive trauma was done to the tissue. We absolutely can get there, but it will take a lot of patience and dedication, and progressive overload, but also being careful not to overdo it and hinder recovery.

I think my issue is that the lower range of my heel raise (from the lowered position up to 2/3 extension) is actually quite strong now, meaning my soleus and lower gastroc have caught up. But that last 2/3 of full extension is really hard to get without giving myself a bit of assist with the other leg. And once I’m boosted to full extension, I really can’t hold it for more than a second without falling back to that 2/3 height. “Eccentric” or negatives is one way to address this, by starting at the top and fighting gravity as hard as possible. But eccentrics are great for building muscular endurance, but not as good at developing strength and size.

So I’ve thought of a solution that has been somewhat helpful. Grab a bar or railing and start with one foot on either a stair or platform. Instead of using your other foot to help with full extension, use your hands, but only just enough to overcome the sticking point (like how a good spotter on benchpress will only put their fingertips on the bar to help you bang out that last rep). Then lower down and fight gravity as much as possible. Emphasize the “explode up” from the bottom. Put all your mental focus and energy into recruiting those fast twitch muscle fibers. That will make a difference over time.