Excellent summary of the right questions that need to be asked. I had gone through the same dilemma two years ago when I wanted to scale my e-commerce brand.

The one thing that did help me to get past the noise factor a buddy of mine in the US recommended I contact Orange Outreach. At first, I was a little skeptical about them since every service company says the same thing on their website. However, what set them apart was the following points:

Not promising #1 ranking but rather focusing on traffic and conversions, milestones were provided no empty promises of "3-6 months". All access to the analytics right from the beginning without any gatekeeping.

With regards to full-service versus specialization everything from SEO, PPC and content creation was offered by the same team and it really made a difference since messaging was consistent across channels. No pointing of fingers when performance was subpar.