Great, thanks for your continued help. Using the info from you link I was able to configure MacOS mail so that it uses the 'Spam' folder as the 'Junk' folder. Nothing changes server side, its more of a graphical change so that "Spam" looks like its called "Junk" and is positioned up the top of folder list next to the inbox and has a custom icon.
So all good there.
Unfortunately iOS doesn't have any Junk mail filtering capacity so its not possible so assign the Junk folder to anything. Drafts, Sent, Archive and Deleted Messages are all assignable however, as you say.
So this all ends up with there still being the same 'Spam' folder server side / with Webmail, the same with iOS Mail, and MacOS mail graphically pretending 'Spam' is called 'Junk'. Not good.
So, as far as I can tell, It looks like you're out of luck if your run iOS. The problems I am having here will have to be solved server side. If Dreamhost can add the ability to designate another Spam folder, or maybe just add the ability to automatically forward the contents from 'Spam' to 'Junk' that would fix it.
Having iOS users unable to properly integrate with the Dreamhost email system is a big problem.
P.s. I have configured a message filter on Cpanel to move any message with 'SPAM' or 'Spam' in the header to the 'Junk' folder. Not optimistic about this working though.