I’m a little confused. Rootless mode means you run the docker daemon as a non root user while processes in the container can still run processes as root from their point of view, except that is not a real root on the host under the daemon. That is for protecting your host machine as Docker running as root would have permission to mount system folders and if you don’t trust the image you use, or if you want o make sure not even bugs or hacked software can harm your host, you can use rootless mode.
But I don’t see wh you would want to run each container in a separate rootless docker daemon if that is what you meant by “separate user per each LXC/Podman/Docker container”. Than using Docker compose has a small benefit and your containers could not even communicate with eachother on Docker networks.
If you mean running processes in containers (rootless container) as a non-root user, that is generally recommended unless you need something in the container that can be executed only as root.
Note that rootless mode has its own limitations so whether it works for you depends on your project
https://docs.docker.com/engine/security/rootless/troubleshoot/#known-limitations
Could you clarify what you mean? rootless docker indeed runs under your user’s systemd, but containers are not “using” systemd unless you install systemd in the container which is not recommended usually and could be tricky