I moved to Proxmox a while back and it was a big upgrade for my setup.
I do not use VMs for most of my services. Instead, I run LXC containers. They are lighter and perfect for individual services. To set one up, you need to download a template for an operating system. You can do this right from the Proxmox web interface. Go to the storage that supports LXC templates and click the Download Templates button in the top right corner. Pick something like Debian or Ubuntu. Once the template is downloaded, you can create a new container using it.
The difference between VMs and LXC containers is important. A VM emulates an entire computer, including its own virtual hardware and kernel. This gives you full isolation and lets you run completely different operating systems such as Windows or BSD, but it comes with a heavier resource load. An LXC container just isolates a Linux environment while running on the host system’s kernel. This makes containers much faster and more efficient, but they can only run Linux. Each container can also have its own IP address and act like a separate machine on your network.
I tend to keep all my services in lxc containers, and I run one VM which I use for a jump box I can hop into if need be. It's a pain getting x11 working in a container, so the VM makes more sense.
Before you start creating containers, you will probably need to create a storage pool. I named mine AIDS because I am an edgelord, but you can use a sensible name like pool0 or data.
Make sure you check the Start at boot option for any container or VM you want to come online automatically after a reboot or power outage. If you forget this step, your services will stay offline until you manually start them.
Expanding your storage with an external SSD works well for smaller setups. Longer term, you may want to use a NAS with fast network access. That lets you store your drive images centrally and, if you ever run multiple Proxmox servers, configure hot standby so one server can take over if another fails.
I do not use hot standby myself. My approach is to keep files stored locally, then back them up to my NAS. The NAS in turn performs routine backups to an external drive. This gives me three copies of all my important files, which is a solid backup strategy.