I'd only need to get one certificate for Nginx and another for the SMTP server, which are two commands with Lets Encrypt. And setting up nginx as a reverse proxy for an application is as easy as adding two lines to a configuration file. It's far simpler, easier and better documented, and if any problems pop up it will also be easier to debug. You only need to know nginx to solve issues and add things: with the k3s setup it looks like you need to know nginx and Kubernetes.