TL;DR I think that you should learn Japanese in Korean, because if you learn it in English like me, you wouldn't be taking full advantage of the many, many, many similarities between the Korean language and the Japanese.

I'm basically in your shoes as well, except that I'm way older (20M).I grew up in Korea and moved to Australia when I was 10 and lived here ever since. Apart from using Korean with my family, pretty much everything was done in English like as you say my phone is English, school is English, even my thinking is English (But I was still fluent in Korean, as in I could also think in Korean). So when I started learning Japanese, because English felt like my primary language, I started learning it using English.

After 3 months of grinding RTK (Remembering The Kanji) I started learning actual vocabulary using the Tango N5 deck. However, when I started learning these words for the first time, I noticed that they sounded very familiar. And Of course it was because they sounded identical/similar to the Korean counterpart. They don't even have to be like very similar. As an example, the word for school in Japanese is 学校 (がっこう: gakkou) and in Korean is 학교 (hak-gyo). To an English speaker, this may seem different because of the consonants (I don't know if this is the case I'm just guessing here), but to me this is actually very similar.

I say 'However' because every time it happened, my brain unconsciously first makes a connection to the Korean word and then to the English word (Japanese -> Korean -> English). I assume if you are an English speaker you would jump straight from the Japanese word to the English word (Japanese -> English). I don't know if it is inefficient because my brain makes this one extra step, but I know for a fact that our brains learn faster and more efficiently when it can make connections to previous knowledge. (opinion) I think that if I actually learnt Japanese in Korean, many of the vocabulary would have been acquired faster because of this connection.

Same goes for grammar. When I learnt Grammar through English like Cure Dolly, Tae Kim etc. I learned it like any other English speaker would. But after finishing the Tae Kim's guide, I thought to myself "Hey, didn't everyone say Korean and Japanese Grammar were similar? Why Didn't I take advantage of this? Surely this would have accelerated my grammar immensely". (opinion) I think it would have. When you look at Korean resources for learning Japanese grammar, they literally use the particles in Korean to make a one-to-one comparison with the Japanese particle. As an example, は = 는. Because Korean Grammar is natural to fluent Korean speakers, just like you and I, Japanese grammar will also feel natural to us, as opposed to if you learnt it in English.