Encouragement / arguments for OPOL method? And media-related questions

My husband is half Japanese and is, while not entirely fluent, highly proficient (like N2 or -3) in Japanese. We planned since before having a baby that he would teach our children Japanese. We live in the US and I speak English. We never looked into how to specifically do this, but since our child was born (he's about 8.5 months old now), my husband has spoken mostly (but not entirely) Japanese to our baby. Two days a week, my husband stays home with our son, and on those days he speaks exclusively in Japanese. However, when I get home from work/if we're all together as a family, I'd say he speaks more like 60% in Japanese to our baby. I think that's still a great percentage, but I am worried that it may not be enough to get the minority language to stick. My main concern is that, starting in August, I am leaving my job to stay at home with our son full-time (I currently work two days a week, which are the days the baby and my husband have their immersive Japanese). I am encouraging my husband to try the OPOL method, but I think sometimes it's exhausting for him. I totally understand that, but at the same time, I know he wants bilingualism for our son even more than I do. My main question is whether anyone can give me some compelling arguments, anecdotal or research-based, for the OPOL method that I can pass along to him?

My second question is how I can best support the learning of the minority language. I've advertised a little for a Japanese-speaking babysitter so that our baby can have more exposure, and I speak a sprinkling of Japanese and reinforce that vocab myself, especially when we're reading books I know he also reads in Japanese with his dad. I also would love for our son's grandma and grandpa, both of whom are fluent in Japanese, to speak only Japanese with our baby -- they live a few hours away, but we do see them every couple of months. I think at the moment they don't want me to feel excluded, so some persuasion I can share with them to encourage Japanese-speaking would also be great! While we don't do screens and stay away from toys and books that make noise, I'm also looking now for books that "speak" Japanese so I can have that external support when my husband is not home. The HabbiHabbi books look great, for instance, but unfortunately there's no Japanese version.

Third question -- I have been passively playing Japanese conversation videos on Youtube in the background at least one hour a day while the baby is playing. We don't do screens. Is this type of exposure useful in your experiences?