Aside from the RKE templates add_on field, which looks like you can set raw yaml, is there any way to default add a helm chart for new clusters? For instance for our on premise clusters we are looking at doing metalLB so people can expose services with HA without having to manually deploy a chart and configure after a cluster is created. I'm having trouble finding a way to give users the ability to spin up a cluster with this built in. I do see https://github.com/rancher/rancher/issues/27627 which would be super helpful, but was wondering if anyone had any workarounds in the meantime.

Rancher doesn't have a feature like that, but you could deploy the same chart across multiple clusters with Fleet if the Helm chart was in a Git repository. (https://fleet.rancher.io/examples/)