there are many ways to install Nextcloud. You can perfectly install using docker-compose e.g. following Docker .examples this advanced guide Nextcloud docker-compose setup with notify_push (2024) or any other tutorial on the internet. you’ll find required information to connect USB drive using “docker volume mount” internet search. Saving pictures from mobiles is hard to setup using “local” system I would recommend setup right way from the beginning - using reverse proxy, external DNS record and valid TLS certificates - everything else cause too much headache. Don’t forget to implement right backup as single external USB drive will fail sooner or later.