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.

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.

Setting up a full k8s cluster (vanilla k8s, not k3s etc.) and running most of my self-hosted apps in that cluster caused me a lot of headaches but also got me an immense amount of knowledge and experience.