Don't leave until you have something lined up. Plain and simple.
So you're afraid of joining another company?! Why? They'll be friendlier, as the company culture is way superior, they'll train you better, you'll be working on technically more relevant stuff (good for career longevity), your resume prestige will be on another level (i.e. you'll get hounded on Linkedin with great offers), you won't be under constant threat of being layed off, and your compensation will be 1.5-2x more. Sorry, am I missing something why you should be afraid? LOL. Come on, snap out of it don't be a punk. Have confidence in yourself.
Start prepping, and when you're 100% prepped, start applying. Your next job will only be as good as your prep. You'll need a good 4-6 months Leetcode to pass the FANG interviews.
Don't discount yourself either! The fact that J hired you means you have a good brain/resume. Getting an interview with FANG ain't that hard at all! They hire all the time! You WILL get your interviews! But way more important is that you prepare like a MOFO! You WILL get challenged, but no mental achievements above what you've done in the past. The rest will fall into place.
Report back when you hit the home run!
Good Luck!