I wasn't allowed to make my sixth subject as CSL 3/4 at school but take my SACS and exam at my XJS.
Why can't you take it at your school? Tbh, I'd rather take it at my school than at an external school (despite XJS being very highly-rated)
When I was in Year 11 I also did 2 3/4s (one of them being Chinese which my school does not offer). My school was like that too where we had to take 6 subjects in Year 11 at school no matter what we do externally (I think literally nearly every school is like that though?) and I saw it as a good thing because you got to learn more. If that rule wasn't in place I probably would've done 5 at school but in the end I'm glad because that "extra stress of a 1/2 subject" made me work better this year and I picked a subject I was interested in. Year 11 is undoubtedly one of the toughest years (tougher than year 12 imo
) and doing more subjects in Year 11 will definitely prepare you for the following year
I don't think any 1/2 subject is easy but it depends on the person. Do what you're interested in (and not what's 'easy' - I have no experience with BM/PE/Food Tech unfortunately but personally they sound quite difficult). Even if you do come off as immature or lazy that's fine, lots of kids at my school did that as well in your situation (and some of them were just lazy full stop).
If you
really badly don't want to do 7 subjects, you can get your parents to argue, complain about how stressed/depressed you are or drop it like 3 weeks into next year (only if you're desperate).
Good luck though!
I can understand why you're pissed but feel free to disagree