This prompts a reflection about, as an industry, we should make a better job in providing solid foundations.
When I check tutorials on how to drill in the wall, there is (almost) no warning about how I could lose a finger doing so. It is expected that I know I should be careful around power tools.
How do we make some information part of the common sense? "Minimize the surface of exposure on the Internet" should be drilled in everyone, but we are clearly not there yet
As a society we already have the structures setup: The author had been more than welcome to attend a course or a study programme in server administration that would prepare them to run their own server.
I myself even wouldn't venture into exposing a server to the internet to maintain it in my freetime, and that is with a post graduate degree in an engineering field and more than 20 years of experience.
I find this to be extremely sad.
Unlike welding or diving, there is no inherent physical risk to life and limb to running a server. I should be able to stand up a server and leaving it running, unattended and unadministered, and then come back to it 20 years later to find it happily humming along unpwned. The fact that this isn't true isn't due to any sort of physical inevitability, it's just because we, the collective technologists, are shit at what we do.
good news! there is no inherent risk to life or limb because you left your server exposed. As OP discovered, you might come back to find it running a crypto miner. and that's just really not that big of a deal. maybe we're not all shit at what we do, but rather we have appropriately valued the seriousness of the risks involved, and made the decision that locking everything down to be impossible to hack isn't actually worth the trade-offs to usability, convenience, and freedom.
you can leave your iPad running, unattended, and unadministered for 20 years if that's what you wanted, and come back to find it un-pwned.