I've had peers in your same circumstance and they scoring in that same percentile range (Low C to Low B) and some of them chose to drop. I spoke to all of them on why they dropped and here's what I found.
These are the main reasons I found:
A) Ext-1 took too much time for the results they yielded. (Even students who scored a decent B [70-80]):
This just comes down to poor allocation of time and method but its understandable as it does indirectly eat up the marks for other subjects.
B) Poor Marks:
I don't believe one bad assessment should determine if you drop a course but it is what it is.
C) Focus on other subjects (sciences, english, humanities etc):
This is the most acceptable and valid reason as long as its backed up and you follow through with a proper structure and plan on how you will reallocate your time to being more productive in other subjects.
This is my personal opinion once again and I recommend speaking to your ext-1 teacher and someone more experienced however from discussing with students in your place, I believe you should try your hardest; research studying methods (kolb's experimental cycle?), implement study methods (timetables, goals, taskbars etc) and find other resources and material to learn from (eddie woo, khan academy) and dedicate more time to ext-1 than normal just to see how you go. If you do see a good improvement in your prelims or next assessment task (B grade or above) then it definitely pays off, as I stated in my previous post, ext-1 is super helpful in learning new techniques to apply in advanced and gives that edge.
There's no point in sticking with ext-1 if it has no prospects for you or it does not interest you at all, if its a burden for you then its best if you just do advanced as ext-1 requires more abstract thinking and effort whereas advanced is more basic and repetitive.
All the best!