True story:
I knew a guy who didn't do GMA in year 11 & he got a raw 49 in spesh.
Of course, that comes with hard work, but it's not iMPOSSIBLE.
I would recommend just jumping into the spesh 3&4 textbook because it has everything you need to know, whereas for the GMA textbook, there are actually things you learn which are not assessed in the 3&4 course.
But definitely, you can work through that over the holidays/ get a tutor & yes, worked solutions would be helpful.
In my opinion, I would choose spesh>further anytime.
Why?
It's easier to get a scaled 40/50 in spesh than in further.
In further, you get scrutinised and traps are set everywhere & it's so easy to screw up in the exam if you're not careful.
For spesh however, I felt like 70%++ of it were (basic) textbook questions....
& they don't really trick you in spesh because they acknowledge that it's already hard to grasp the concepts..
Idk think about this.
If you are a very very very careful person, then okay, you can go for further.
If you enjoy maths/are considering engineering, then you should go for spesh.
But ultimately, doing spesh alongside with methods will definitely benefit methods.
Whilst you might be churning papers after papers for spesh & doing close to none for methods, you can still perform well in methods.
Because besides probability, spesh does overlap with methods...
Good luck!
Even though it's gonna be a tough year for you but prioritise well & have good time management
!