I would learn the following at some point (not in order):

• nftables - specifically how tables/chains/rules interact and how to use them correctly (this is a learning process)
• ssh - using non-default port key-only authentication.
• custom routing - more advanced, but is very handy if you plan to have services talking to each other in non-typical ways.
• openvpn and WireGuard - pros/cons, what they do and how they can make connecting to things more secure.
• podman - docker alternative that is imo more secure and open. It can run services in a non-root enabled way (rootless) which is great with you get it working, but can be hard to get there.

Above are more on personal preference in some cases. There are so many options to do things in the Linux world.

Be careful relying on AI to do this stuff for you. I’ve reviewed some generated nft configs that left things wide open in some cases, or added needless fluff that worked, but caused bottlenecks. Better to know what it is doing, so you can catch when it is confidently incorrect.

Ofc, if you want to go back to basics, you should first dive into the file system, data storage, permissions and ownership. Without those, the rest might not be able to protect you no matter how well they are setup.