I have definitely had them thicken into sturdy stems after starting spindly. Do you top them after the 3rd or 4th set of true leaves form? That will make them thicker and bushier with more flowers (but later)

Yes, I definitely top them at the 3rd leaf level. I have noticed their stems thicken considerably within 2 weeks of being topped too. But that's usually with tubers that grew fairly thick stems to begin with. Many of last years tubers that I over wintered are growing thin, spindly stems. I hope topping them, and time will rectify the problem(if they even survive this latest deluge of rain!)