Dilletante already exists to mean someone who doesn’t do something with seriousness and amateur doesn’t carry the same connotation as amateurish anyway so you don’t really need a new word.
The term 'nerd' needs to complete its rehabilitation like 'geek' has the last 20 years. It's the most concise term I can think of when describing someone who is enthusiastic, focused, and knowledgable on a subject. I think it's a badge of honor
"Amateurish" or "amateurishly" feel damning and assertions about a certain absence of quality or attention to detail.
Describing someone as a "total amateur" feels a bit like calling them a hack.
This needs a separate word or concept.