The best team composition I have ever worked on looks like this:
Product manager: Has <=3 teams knows company product / strategic goals / customer needs. Sets high level goals, updates those in response to scope estimates, coordinates with product org.
Eng manager: Highly technical, has institutional knowledge of codebase + understands the politics of the org. Has <=3 teams. Sets tone, handles dev career growth, mediates/prevents conflicts, recruits, shields devs from institutional nonsense.
Senior Dev / Tech lead: Trusted by eng manager to independently make arch level decisions. Can mentor / peer with / support other devs.
Dev1: Specialist in one relevant aspect of stack, but with general knowledge (can pick up any ticket if needed)
Dev2: Same as above, different specialty
QA: Half-time dedicated QA resource
UI/UX Designer: 33-50% time for team.
Project Manager: Half-time dedicated resource. Owns Jira, scrum master etc as noted above.
This group is flexible, resilient, and plays to people's strengths. If you don't have a project manager then yes it can be left to the eng manager or tech lead, but it's not ideal.