Needing to convert yaml to hcl seems like a burden. A killer feature of HCL would be if it supported syntax highlighted yaml. Or if the HCL manifest was in json format, it perhaps could be easily then converted to yaml as the "human readable" format, then converted to json and tf apply.
You can use YAML files with kubernetes_manifest resources if you like, though you'll probably run into issues that the bespoke resources (e.g. kubernetes_deployment) handle for you.
the helm provider also exists and is a heck of a lot less trouble than converting yaml to hcl.
Yeah that's definitely the way to go if there's already a helm chart. If it's just raw manifests I'd rather convert to HCL than build a helm chart to feed into TF, but that's just because I know TF better, most of my configuration is already there, so I'd prefer to keep abstraction boundaries small. But it's nice to have the options.
It’s fairly simple to set up a basic helm chart around existent yaml specs, then progressive parameterization of that yields a nice managble object.