welcome to the forum @Gremlin 
the guide looks valid the only issue I could imagine - you missed the MariaDB config for Nextcloud container on first start… Nextcloud’s Docker container only consumes some variables on the very first start and once this config was applied it never consult ENV variables again see Inventory and clarify how each auto-config value is handled [WIP] · Issue #2224 · nextcloud/docker · GitHub for details.
It is crucial to create all the compose.yml and .env file before you start the container! Depending on the current state you could start from scratch or migrate the DB using the occ command.
look at
- Nextcloud docker-compose setup with notify_push (2024)
- Nextcloud docker compose setup with Caddy (2024)
for more complex compose setup examples. depending on your goal and knowledge AiO could be good turnkey solution for an easy start.