I have created a setup on Debian 11 with unpriviledged LXC containers. I only use LXC, not LXD.
For autostarting the containers I use systemd as described here: ubuntu 14.04 - How to auto-start unprivileged lxc containers? - Server Fault
So i created a user service for system d and that works partly. Strange thing is I have 4 containers and only three are started, when i change the boot order of the containers a different one is missing. i can start it manually without problems.