Early in BJJ I noticed the benefits of flexibility for safety and to expand my game. The most helpful advice I received was to be patient with my body when it comes to flexibility. As a bigger person, you are probably used to working on strengthening where you can see significant progress in say 14-30d. Unless you have freak genetics, think about flexibility on about a 120d schedule when setting comparable modest goals or expectations. Be patient. Tendons etc take longer to grow/heal than muscles, that's just nature. And make sure you aren't doing things to/through pain, just mild discomfort, keep your joints safe.
Secondly, because of your goals to improve and expand your BJJ game, make sure you don't just work on extending your flexibility (ability to contort your body further) but also extending mobility (ability to move your body over a larger range). The two don't always go hand in hand.
There are plenty of YouTube videos I've found and instructionals (if you care then I can give some links from my collection). Yoga can be a great source but they don't always include the mobility with the flexibility so I suggest being picky or making sure to mix things up. Try to find instructionals that give you stages and progressions so you don't stagnate.
I'm a bigger guy. Despite being very flexible in some ways (e.g. for high kicks), I had uncomfortable limitations when it came to shoulder and hip flexibility. Over many months I gradually became quite flexible in my hips (went from not liking to cross my legs even to being able to comfortably do a half lotus and am working on a bit more) and my shoulders are now flexible enough to ensure that my training partners need good technique to finish arm/shoulder locks.
One last important specific tip I found helpful. When doing the dove hip stretch or half lotus type hip opening stretches, engage your foot (ancle at 90deg or less). It helps to keep your knees safe. In the dove stretch, if your knee tends to hurt, it's because the stretch isn't coming on correctly, it should be in the hip not the knee. Prop yourself up, try to hold good posture, one leg extends back, and make sure the bent leg is perpendicular to your body with the foot engaged before lowering down. You won't be able to go down as far but your knee will thank you.
