Higher/advanced higher subjectsWatch
You can choose basically anything if you have a good idea for the layout of the dissertation / what things you'll look at. There's a list from the Shetland Library I think that has a good list of books based on theme which is good. Mine wasn't on there, though. I'm doing 'Tell my Horse' by Zora Neale Hurston, but one of her other books 'Their Eyes were Watching God' was on the list.
You could ask your school about going to a different school to do advanced modern studies. I’m doing it this year and really enjoying it, it is quite a step up from higher but if you get a good teacher it’s super interesting and so is the dissertation! Also unis don’t really care about advanced Highers so I’d recommend doing 1 like English as you said maybe not maths as it’s not related to the course you want to do. I do history as well but that’s just because I like the subject. So instead of doing 2 or 3 advanced Highers it’s probably better to do more Highers as they are actually needed for entry requirements. Idk if that makes any sense lol
Thank you for your help!