I don't think it necessarily needs a monorepo, but having every package on Salsa (Debian's GitLab installation) would help.
I wish they went all in and used GitLab Issues for bugs and GitLab CI/CD to auto-build packages for both validation and pushing new packages into the Debian repositories.
I love using Debian, I care deeply about Debian Policy and Debian's procedures, I enjoy many aspects of the Debian community, and I'm also well aware of where Debian has difficulties.