I strongly recommend you to learn Hangul and watch content at the same time.
Start off with this online course called First Step Korean by Yonsei University on Coursera. Yonsei is one of the top universities in SK so you can trust their courses. This helped me a lot and I will be revisiting this course to brush up on my Hangul. Maintain a Hangul workbook and regularly practice your alphabets and pronunciation. It is important to do so. Practice your reading skills on Twicetagram posts. Take it easy with the course. Don't finish all the modules in one sitting and then pat yourself on the back. Do it as it is intended, week by week. Revise daily, even if it means spending a mere 15 minutes on it for the first week.
Since you mentioned that you want to pick up vocabulary through my technique of watching a lot of K-content but specifically something that has TWICE in it, I would suggest to watch their entire upload history on the V LIVE App. Start off with the TWICE TV in chronological season order. For the first watch, watch the episode with subtitles on. Then watch it again with subtitles off. When you are watching it again with subtitles off, try to keep track of their conversation and try to understand what they are saying. You will be able to pick up a few words they repeat because you have watched the episode one once with subtitles on and once with subtitles off. Keep doing this for anything TWICE related that has subtitles. Watch it twice, once with subtitles on and once with subtitles off. But concentrate on what is being spoken, don't get lost in their visuals or whatever it is that you like about them.
If you want to watch K-content other than TWICE, check out gooddrama.net. They have many Korean subbed movies and dramas.
After completing the First Step Korean online course, I suggest you enroll yourself in the Learn To Speak Korean 1 online course offered by Yonsei University on Coursera. Don't jump the gun and enroll until you have finished First Step Korean.
Contact your local Korean Consulate if you currently live in the capital city (or a major city) of your residing country. Ask them if they have any courses on learning Korean. Or try contacting them via mail. Consulates will be more than happy to help a foreigner who is showing interest in their language and culture. Simply ask them if there are any textbooks or beginner material that they can recommend to someone who wants to learn how to read, write and speak Hangul.
Subscribe to /r/languagelearning and explore the sub. Search for posts related to Hangul.
If you're really interested in learning Hangul and exploring Korean culture, I would recommend taking a gap year after your higher secondary studies are over and enrolling in the Yonsei KLI day course (not the evening one). If I had gotten into KPop as early as you have and had I found out about the Yonsei KLI course before my ill-fated undergraduate studies began, I would have definitely taken a gap year and left for SK to attend the course.
Good luck and Godspeed.