Experimenting with VMs is the way forward.
Basic networking knowledge is vital. And being able to configure your own firewall(s) safely is an important skill. Check out something like Foomuuri, or Firewald. Shorewall is brilliant for documentation and description of issues (with diagrams!) but it does not use the newer Linux kernel nftables and is no longer actively developed.
Go for it with Nextcloud.
I would also recommend at least having a shot at setting up an email server, although I would recommend pushing through to a fully working system. It is possible, and is very satisfying to have in place. The process of setting one up touches so many different parts of internet function and culture that it is worth it even if you don't end up with a production system. The Workaround.org ISPMail stuff is a good starting point, and includes some helpful background information at every stage, enough so you can begin to understand what's going on in the background and why certain choices are being made - even if you disagree with the decisions.
Python is great for server admin, although most server config and startup shutdown snippets are written in BASH. You will no doubt have already begun picking that up as you interact with your VMs.