I find your example simple and easy to understand and I don't see any issues with defining everything explicitly.
I'm using docker swarm/compose extensively for multiple years and configuration can be a bit verbose sometimes but it's very simple system to maintain and write config for.
I think, splitting relatively small configs into smaller bits and using additional abstraction can actually increase complexity of the system compared to having everything in a single file as it's basically write and forget and not something you modify heavily once system is mature.
I'm using docker swarm/compose extensively for multiple years and configuration can be a bit verbose sometimes but it's very simple system to maintain and write config for.
I think, splitting relatively small configs into smaller bits and using additional abstraction can actually increase complexity of the system compared to having everything in a single file as it's basically write and forget and not something you modify heavily once system is mature.