Why is sales demotivating? If the product and market are right, it should be exciting. I love sales because it's like talking to fans.
Designers are often needed very early. Pay might be a little low early on.
Product is also vital but often management/founder tier that early on.
Testers are often vital in the early stage, sometimes second or third employee. It's not all bashing buttons. Testers often follow the sales team and act as customer service. It's important to understand UX/customer flow and can be an entry point to Product.