Strategy is a term that is misused often. A strategy is making a clear choice of between options. As a metaphor, I like to use a vacation as the example. The strategy is choosing where to go. The tactic is the means of transportation. Here, you need to evaluate if the agency you choose has a clear understand of where you are going and how it can deploy its tactics to help you win.
Sometimes marketing is an investment. Sometimes programs work. Sometimes, they fail. Regardless, you need a measurement plan in place to know which to kill and which to grow.
Learning is part of any marketing campaign. It is how you improve and grow. The market is constantly changing. You will be best suited to meet that change if you constantly testing.
Digital marketing is a diverse field. One group specializes on web design and another on social. If you need a specialized agency for each, the costs add up quickly. Each have overheads they need to cover. This is where a full service agency can help. Those overheads are shared the different disciplines. Now, one caveat, it is important to know that the full service agency can provide the same if not better quality as the specialized.
Large company solutions or carbon copies of other campaigns rarely work for small businesses. If you are seeking marketing help, make sure that the firm you chose has the experience in small business.
If a marketing firm will not share some of the risk with you, then they are not as vested in the outcome as you are.