The way the tip of your thumb points up, makes me think you are digging in real hard with your last thumb knuckle. I used to do that. Now I use the pad of my thumb, which lets me manipulate the nose angle better. If you do the hand gesture for "money" while gripping your disc, you'll see what I mean.
Have you read about the hammer drill at all? Doing any kind of disc golf grip is kind of pointless unless you get how the disc is supposed to pivot around your rip finger before it comes out of your hand. It sounds like it's slipping out and random times instead of pivoting and ripping out of your grip. I'm not saying you don't get that yet, but I sure didn't when I started playing.
The reason I bring it up the hammer drill is if you grip the disc with only your index and thumb, and do something kind of like the hammer drill, you'll immediately see how a proper disc golf grip works. The rest of the fingers just stop that pivot from happening too soon, and make it so when it does happen, it happens more suddenly and with greater force.
Anyhow, if you haven't yet, I'd head over to the secret technique thread, and watch the "yo-yo" video, and try it with a two-finger grip. Once you get that feeling down, you can try to incorporate it into your throw and might hopefully help with your release problems.
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