Nice scope! I had a similar experience with using Claude to automate circuit design/simulation/optimization and found that they are not good at it. They are surprisingly good at taking raw files and describing what is in them, but they fall apart when trying to do anything other than design the simplest circuit. I think it is because they have no concept of the physics behind a circuit, so they cannot make changes that a designer would make. For optimizing a circuit using, say, an EM simulator, they don't know what to tweak and how to tweak it. In the end, I had to write a script to talk to the simulator and create a config file that specified the bounds of the simulation: step size, optimization algorithm, min, max, etc. Only then could I use an agent to call the script to optimize the circuit.
Yeah, taking the spice list as the starting point works much better, imo. I also prepopulate the CLAUDE.md file with some information like the pinout/pinmux of the MCU otherwise claude might run in circles trying targeting the wrong pin (to be fair that also happens to me, lol).