How to stress-test and pilot an online cognitive-load decision-making study?

[SofiaBT]

I'm a psychology PhD student designing my first independent experiment on cognitive load and decision-making, and while I have my hypothesis, I'm second-guessing my experimental design at every turn. I'm worried about confounding variables, especially with an online participant pool, and whether my manipulation check will be strong enough. For experienced researchers, what practical steps did you take to rigorously stress-test your design before running subjects? How did you effectively pilot your study to catch issues with instructions, task difficulty, or measurement validity, and what resources or checklists did you find indispensable for ensuring methodological soundness?