if youre a strong math student, which it seems you would be since youre doing methods and spesh, further won't be too difficult for you.
i probably put less than 50 hours into the subject over the entire year last year, including time doing the assignment and worked on spesh/chem in class and came out with an okay respectable score (over 40).
if you're aiming for high 40s though, of course you're gonna need to put in more time.
but imean, you may as well take further since it doesnt require that much effort. if you go in with the intention of it being an increment subject (since only two maths can be in your top 4 anyways), and don't put much time into it/worry too much about how you do in it, it won't add very much to your stress levels and it can still boost your aggregate by a couple of marks. besides, since you've already done unit 3/learned half the content, why not?
oh and i also did further along side five other subjects.