I went to look up if Dreadnought had linked origins with altered spelling but no, the "nought" is what it is, "nothing". which is weird because surely a big ominous ship would be a Dreadnaut

this is an excellent point! it's not instilling dread, it's just fearless!

A Dreadnoughtnaut is a sailor on a Dreadnought, which is commonly shortened to Dreadnaut for brevity.