I'd like to have a better balance with all my studies, not just English.
I'd like to focus more on my other studies as well as English.
この場合、次のような言い方ができますよ。
ーI'd like to have a better balance with all my studies, not just English.
「英語だけでなく、（他の勉強も）よりバランスよく勉強したい」
not just で「…だけでなく・…に限らず」
ーI'd like to focus more on my other studies as well as English.
「英語はもちろん、他の勉強にももっと集中したい」
as well as で「…はもちろん・〜も〜も」
ご参考まで！
カナダ 
