For anyone searching the threads for an answer to this riddle - there is a hidden “Advanced Settings” mode that can only be accessed via heading down to the bottom of the root settings screen and tapping "More."
This then allows you to change the settings “level” to advanced, which in turn allows you to see the "Hide All Seasons option.
It’s not an OSMC issue but a Kodi one, but it would probably be useful to either get rid of the “Advanced settings” menu as it doesn’t really add many settings (none of which seem potentially harmful - or indeed advanced in this case) or at least add a dialogue somewhere saying that there are advanced settings available, as if you are just looking at “Library” settings it doesn’t mention there are hidden options, hence me pulling my hair out in confusion.
Anyhow, thanks again for the prompt responses - I think I now have my setup all organised exactly how my OCD brain requires.