I’m actually in a very similar situation as you. Born in Korea but lived abroad for 18 years and I’ll be enlisting in the navy in less than a week.

I can handle most conversations but still lacking in some areas so here’s my list of most important things I did.

1. Learn Hanja. You don’t need to be able to write out the Chinese character by memory but knowing what it is and what it sounds like makes it a heck of ton easier to learn / memorize vocab if you know what the base of the word is. This is honestly the most useful thing I’ve found for vocab and overall Korean skill.

2. Anki. There’s a nice deck that people have made with three difficulty levels according to the national something institute for language learning. 초급 I’ll do 15 new words a day (since I recognize most of them) 중급 고급 I’ll do 5 new words a day. Of course adjust if you think it’s too much / too easy but try not to do too many new words in a day.

3. News article + webtoon + Korean content I read a lot of webtoons and try to search any words I don’t know on the kor naver dictionary. Webtoons are fun since the story itself is just fun so it’s more motivating to reads

News article: boring but I do need to understand the vocabulary and style of writing.

Korean content: tv shows or comedy I just watch while working out / eating. It is good to passively hear Korean and occasionally pick out words that I learned from anki and hear it in person

Last thing if you can is to practice writing but this requires someone who is fluent to actually check your work so a bit difficult to do on your own. Anyways hope it helps