Cannot say the same with regard to their Pad Thai. But Pier 21, as you suggest, is a good place to start, as they have a good range of inexpensive meals, including the option of being able to choose from different toppings on rice, for a one plate meal.
If just for toppings on rice, my favourite is near the entrance of Ratanakorn Market, in line with the entrance of Mr DIY. One plate meals from 25 baht. Freshly prepared and tasty. Not sitting there all day, but smaller batches regularly replaced.