Good question! Think many CS organizations struggle to define clear "swim lanes" for CSM and CSA/Solution Architects. We all have examples of CSMs playing the Solutions Architect role and vice versa.

As a rule of thumb: I would define the CSM role having a broader responsability, particularly between the transition from implementation to expansion/scale up of the account. Depending on the maturity of the organization, when there's no dedicated implementation lead or PM, CSMs are often expected to manage the project and keep all stakeholders aware of the progress, as well as the RAID log.

Solution Architects are typically much more engaged during the the implementatation phase to ensure, on one hand, that all technical elements are aligned to the use case/s,.but also to ensure that the approach/decisions being made at this stage are future proof and scalable, on the other.

For us, CSMs are assigned to accounts and are responsible for driving outcomes. CSA are more technical and are a resource for CSMs to tap. The CSAs are shared across the entire customer base. They can airdrop in for a quick configuration of project and then pull out. 

We also have Solutions Architects which are a paid resource and would be dedicated to the customer. Their engagements are longer than CSA as well as usually scoped.