The core premise of this counterfactual seems highly dubious. Caesar spent his entire adult life under incredible amounts of toil and stress. He likely suffered from various serious ailments. He was a hard mid 50s. We know what happens to all humans at this age and under these conditions.
I've read a lot about this period and always come away with the impression Caesar seemed very burned out and done with it all. He had achieved everything he had set out to achieve and more. There was nothing much left for him to do, even if he was a "workaholic" that would've surely kept going by default.
It's not unreasonable to accept the theory that he might've even dismissed his bodyguard, and ignored the assassination plot rumors, in hopes of the glorious suicide-by-assassination exit he received. He was clever enough to do something exactly like this and clever enough not to tell anyone about his plan.
The man rolled deep with bodyguards for decades, was highly paranoid about his own safety, and then dropped it all while surrounded by people that we know that that he knew wanted to kill him, and had even tried.
Even the distant German king Ariovistus received messages from Caesar's peers in Rome offering rewards if he was killed. Caesar reported this in his commentaries, and this was years before he had defeated and humiliated huge numbers of powerful people.
Even if the plot had failed, and even if a burned out Caesar is impressive compared to any of his contemporaries, and even if it's unlikely anyone could've beaten him. It still seems very unlikely he could've kept up the pace for another decade, which is what any serious expansion would've required.