K8s really shines when you start hosting more stuff, even on a single node. I definitely recommend giving k3s a try. I wouldn't recommend it for only a couple of services though.
Is it overkill? Yes, applying docker-compose manually also works. But then you still have to make your reverse proxy, your certificate and all your services work together. You can write Ansible for it, but then you end up with a lot of custom code to maintain and you still don't get all the nice features.
For me the killer feature was flux. Your code, configs and even secrets live in git and get autodeployed and autohealed. And it has other features such as operators to fetch helm charts from other repos and apply your config to it.
I would start by moving the services running on the host to a VM, less downtime for those when switching to proxmox.
Also, if possible, address the data issue before migrating. If you can add more disks, you could setup a new zfs pool, ready to be used by proxmox.
And don't forget to backup (to external storage), you never know what could go wrong.