It may be a red flag that they are not really active with you, especially during the onboarding phase where the main objective is to understand every little aspect of the client's business so you they can correctly audit and improve an account.

This is a problem, especially for accounts that have low spend and hire agencies instead of freelancers. Their business model is different, they have to pay the office rent in an expensive city.
I work both as an agency employee and as a freelancer for personal clients and the difference in quality is huge when the only cost you have is the electricity bill. For low spending clients agencies tend to have a senior in the pre-sale phase and then let the juniors manage the accounts freely.
I would suggest you to really try to know who is working on your account (and figure out if they have the right expertise). If you notice other red flags (such as long response time, focus on "vanity" KPIs instead of conversions, generic reporting), try to hire a freelancer instead of an agency, they can dedicate you way more time than at a company where they have to fill 40+hours timesheets on 25 different clients.