If your stuff is all Docker then yeah, immutable makes sense as it makes the entire box declarative and immutable: you can get back the exact same operating Docker environment on the server, and then you can get back the exact same Docker workloads going with the Docker compose configurations.
If you ever need to run stuff you’d run on Debian, you can just shove it in a Debian container.
That said, if most of the stuff is containers, the risk of just the core Debian breaking is fairly low. Pick whatever is easiest for you to deal with based on your needs. Immutable distros have a bit of a learning curve.