A good first Question : for you to suggest the group thinking about when forming into teams is "How will we make sure all teams have the right amount of expertise?"

Explanation/Reference:
Explanation:
Self-organized teams allow teams to creatively solve difficult problems, be accountable for the work they do, and commit to each other and the goals of the team.

As the Scrum Master, your role is to serve the Scrum Team by helping them to work effectively and deliver value. You are not responsible for managing the work or assigning tasks to the Developers. You should respect the self-organization and autonomy of the Developers, and support them in finding their own solutions to the problem. You should also facilitate collaboration and communication among the Developers and with other stakeholders, such as Phil and the Product Owner.

The Scrum Master is responsible for promoting and supporting Scrum as defined in the Scrum Guide. Scrum Masters do this by helping everyone understand Scrum theory, practices, rules, and values.
The Scrum Master is a servant-leader for the Scrum Team. The Scrum Master helps the Scrum Team understand which of their interactions are helpful and which aren't.
The Scrum Master helps everyone change these interactions to maximize the value created by the Scrum Team.

The situation described in the question indicates that there are multiple problems in the Scrum Team that have led to the increase of technical debt and the dissatisfaction of the Product Owner.
These problems include:
The Scrum Team has not used the past Sprint Retrospectives effectively to inspect and adapt.
The Sprint Retrospective is an opportunity for the Scrum Team to reflect on their performance and identify improvements for the next Sprint. If the Scrum Team has not been using this event properly, they may have missed some issues or opportunities that could have improved their quality and value delivery.
The Developers have not been paying enough attention to quality. The Developers are accountable for creating a potentially releasable product increment that meets the Definition of Done and the Sprint Goal. If the Developers have not been focusing on quality, they may have introduced defects, errors, or inefficiencies that compromise the functionality and usability of the product. The Scrum Team is not having conversations around technical debt. Technical debt is the accumulation of work that needs to be done to ensure that a product is maintainable and extensible in the future. If the Scrum Team is not discussing and managing technical debt, they may be neglecting some important aspects of the product architecture, design, or code that could affect its scalability, performance, or security.
The Scrum Master has not ensured that the Scrum Team is being transparent. The Scrum Master is responsible for establishing an environment where transparency, inspection, and adaptation can occur. If the Scrum Master has not ensured that the Scrum Team is being transparent, they may have allowed some hidden problems or assumptions to persist that could affect the quality and value of the product.