I also used howtostudykorean.com at the beginning. The only thing I slightly regret is making so many flashcards for words that I never actually used for months because I was using this website’s vocab. Sure, the vocab is used on the website, but outside of that, there are some words that you just don’t need to know as a very beginner, because you simply won’t encounter them enough.
I also have Korean grammar in use, but I would say it’s not necessarily the best ressource to use on its own, since the explanations are pretty short. I use it more as a reference, or when I want a greater understanding of a grammar point I’ve already come across a couple times. It might be good for you since you’re already familiar with Japanese grammar though.
For video courses, my favorite ressource is Go! Billy Korean. If you want a more structured course, he has a full beginner course for all the beginner level grammar.
What I truly love though are the Livestreams, where he does more in depths explanations about one or several related grammar points for 1h - 1h30. You can watch either the Livestreams or the abridged versions. He alternates between Livestreams for different levels, so even as an absolute beginner there are some you could watch.
Now, for community, Billy also has a discord. And I’m going to be very shameless when I say: join the study group! Currently, every week, we have one drama group, one listening group, one reading group and a B2 bookclub. And even as an absolute beginner, with your experience with Japanese, you could already join and actively participate in the reading group for example. Basically, we read together two stories, one beginner and one intermediate, and we have a list with every vocab word and grammar point explained. So it’s a good way to familiarize yourself with the way Korean sentence structure works, and to learn some vocab and grammar in context.
There’s also another channel that I really liked as an absolute beginner, and this one is more based on learning vocab in context. It’s called Learny Language and it shows you some drama extracts and then focuses on some words. The vocab also mostly builds upon itself, although unfortunately that channel hasn’t uploaded in the past two years so there isn’t that much content.