I let my kids try out YouTube kids and I thought all the content was total garbage. Mostly toy unboxing videos, video game walkthroughs, and creepy knockoffs of cartoons they liked. Deleted it very quickly. We stick to DVDs and Netflix now.
Yes Guided Access is great. I default it to a 20min timeout. So if I let my 8 year old play Minecraft on it, and he runs off to some corner of the house to binge, I don't have to worry about hunting him down. He'll get a nice warning around 3min out, and then the phone will essentially brick itself. It's great knowing that he will absolutely be coming back to me with the phone in 20min.