In terms of core strength, there isn't much work to put in. Six or so months ago, my core used to be one of the weakest links in my body. Now, I can confidently say it's up to speed with the rest of my body. Just do a 10-minute ab workout before going to sleep, or after waking up, daily - don't skip. Take simple exercises like leg raises and crunches for 10 minutes constantly, doing your best to pause the least amount possible.
In terms of your body, it depends on your age. I am a huge proponent for strength training but I'm no specialist or expert when it comes to it, and I don't want to give advice that can harm you at your age. But regardless of age, body-weight-based conditioning is always beneficial. You probably won't gain mass, but muscular endurance is always a plus.
Multiple sets of pushups, pullups, and squats until failure daily might not improve your Judo skills, but they will certainly improve strength. Strength is a skill - a lot of martial artists tend to have rods up their asses about strong people or using strength instead of technique, but it is a valid method, and strength supplementing your technique will make your abilities soar. Get stronger.
In terms of Taekwondo, go back to basics. Take the basic kicks, especially the ones you struggle with, and practice them. Polish your technique, make your weaknesses into strengths. Polish your forms, strive for perfection (although you will never achieve it). But don't be like me. Take rest days and learn from my mistakes - I didn't take rest days over lockdown when practicing my kicks daily, along with stretches and my dojo's Taekwondo Zoom sessions. I was diagnosed with tendonitis in one knee and patellofemoral pain syndrome. I am fine now, but do remember to rest as much as you train.