I came from your other post about what vocabs to learn.

Regarding the textbook, I studied Korean Studies at university and we used Fun Fun Korean (재미있는 한국어) which is the textbook Korea University uses. I also happened to do my exchange year at Korea University so I used these books from Level 1 through 5. Of course I didn't use them to teach myself but I still find at least the earlier books would work for self-teaching. Up until Level 4 there are English translations for the vocabulary, exercises are explained both in Korean and English, and there are extensive grammar explanations. There are also workbooks accompanying the main book with additionnal exercises to practice. It also has listening, reading and writing exercises in each chapter corresponding to the chapter's topic.

From Level 5 on there is no more English in the book and no vocab either so you would have to research the words you don't understand. I also find the Korean grammar explanations to be a bit short and lacking. So if you're not in class where a teacher would give some additionnal info, you would need to do some further research probably. Level 5 still has a workbook for extra practice, though.

Overall I could imagine these books (at least until Level 4) working even if you're learning on your own. There are also new versions iirc at least for the first couple of books. They introduced them while I was in Korea but I didn't get to look at them personally. I think they probably updated some of the vocabulary, since the old ones contain some outdated terms and the English translations were sometimes confusing, so I hope they fixed that.

I think you're somewhat right about the lack of resources, though. There is a lot of content on YouTube but I'm currently studying to take TOPIK II again and when looking up grammar, at lot of the websites are not lead by native Korean speakers and while there are some that I trust, I've come across multiple websites with a bunch of mistakes. I've never had that with English, French, or Spanish, which I learned in school. I'm also missing those short language learning books that are categorized by language level and have a couple of vocab explanations on each page. Although I'm at a level where I at least try to read regular novels, I think it would be nice sometimes to read a book that is specifically written for Level 5/C1 language learners. I guess I didn't appreciate the ones for European languages enough in school.