You’re right to be reluctant to apply everything by hand. K3s has a built-in feature that watches a directory and applies the manifests automatically: https://docs.k3s.io/installation/packaged-components

This can be used to install Helm charts in a declarative way as well: https://docs.k3s.io/helm

If you want to keep your solution agnostic to the kubernetes environment, I would recommend that you try ArgoCD (or FluxCD, but I never tried it so YMMV).

I guess the network will be a bottleneck on Garage too. If you want high performance you might need a hybrid solution, like clustering of stateful apps on local storage as well as periodic full backups on a distributed storage.

Longhorn is pretty easy to use. Garage works well too. Ceph is harder to use but provides both block and object storage (s3).