Is 不仅仅 the correct word?
Yes, 不仅...而且/还是...  is equivalent to "not only... but also...". I think 不仅仅 fits here. Because the author really wants to emphasize the achievement of this coffee shop, which is '首个提出咖啡情绪疗法的品牌'.

Example:


  他不仅是一个模特，还是一个大学教授。


This just tells you that he has two jobs.

But if you say,


  他不仅仅是一个模特,还是一个大学教授


this means that being both a model and a university professor is really impressive.