I'd focus more on the infrastructure instead of choosing what apps to run if you want to improve your knowledge. Running an application is easy. Automation, security, observability, etc. is the hard part.
Learn about Ansible, Terraform, GitOps, backup solutions, logging, network security, etc.
I regret getting a UDM-Pro and recently swapped it for an n5105 OPNsense box. Luckily they keep their value, so I didn't lose any money on the UDMP.