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.

I came here to upvote the post that mentions haproxy, but I can’t see it, so I’m resorting to writing one!

Haproxy is super fast, highly configurable, and if you don’t have the config nailed down just right won’t start so you know you’ve messed something up right away :-)

It will handle encryption too, so you don’t need to bother changing the config on your internal server, just tweak your firewall rules to let whatever box you have haproxy running on (you have a DMZ, right?) see the server, and you are good to go.

Google and stackexchange are your friends for config snippets. And I find the actual documentation is good too.

Configure it with certificates from let’s encrypt and you are off to the races.