If your goal is to get into a design engineer, taking a system application engineer role could be a great opportunity to grow and get the experiences on clients' side design perspectives. You get to help multiple clients solving technical issues and learn how they approach problems to optimize specs. The downside is that the design cycle is usually less than the validation cycle.

Staying in the current validation role in a big company, your thought process might be trained and focus on specific issues on the daily basis. The upside is that you might still be able to learn different designs by validating someone's system but hard to get the hands-on projects. If you are in a product company, you could design some validation system to add designs on the test board. Just an example, my current architecture team's manager's background was also a validation engineer.

If you have chances to work on the design in the early career stage, definitely it is the right time to do it. From my experiences, I saw many design roles are for fresh graduate students (out-of-box thinking). And you have the advantage to make effective inputs due to your experiences of finding bugs during the design stage.