You are missing one of the most fundamental of postures:
Near--side forearm on his hip, the other arm underhooking his far arm (his arm closest to your feet) - when you can't shrimp out with posture under the throat, this posture allows you to reach your hand up (as high as you can towards the ceiling), and then push him DIAGONALLY off you using your bicep in his armpit. This posture works great for two reasons:
1. It removes his elbow from trapping your hips, allowing you to shrimp/snake
2. It removes the weight off your chest, pushing it beside you, which allows you to get on your side easier
If he counters this posture by putting his weight on his far arm, you reach around and grab his gi or his lat, shrimp your hips INTO him, and DRIVE that forearm that's on his hip at a 45 degree angle towards your feet and towards the opposite side he is pinning from. KEEP YOUR HIPS ON THE FLOOR however, a beginner mistake is to try to bridge, which defeats the angle you are pushing him towards.
So, let's say my opponent is pinning me on my right side. I put my right forearm in his hip, and try to establish posture with my left forearm under his throat. When/if that fails, I will BRIDGE to make space, and then slide my left arm under his right armpit. I may need to bridge a second time, or I may not, but I want my BICEP to be in his armpit. I then drive my bicep at a 45 degree angle to my right and over my head, turning onto my right side and moving my hips away from him. If he counters by putting all his weight towards his right side, then I reach my left hand around his back, grab around the outside of his lat (lattisimus dorsi muscle), and then pull his lat and simultaneously DRIVE my right forearm into his hip, pulling/pushing him at a 45 degree angle towards my feet and my left side, as I first pin both my own hips flat to the floor, and then move my hips to my right as I feel his weight reach centerpoint on my hips. With a gi, I would have the options of grabbing his gi or his belt instead of his lat, and optionally gripping his belt (at the side of his hip) with my right hand instead of using my forearm on his hip.
I find that when escaping, it is necessary to be able to alternate directions in the blink of an eye, in order to defeat the pins of more experienced grapplers. If you are locked into the "I've got to shrimp my hips away" mentality, you will only get so far. I'm not familiar with it being called "boxing hands position," so if I'm missing something here that you are specifically focusing on, then I apologize (and please explain). But without the escape I just described to mix in with the postures you listed, I find it extremely difficult to escape against someone with developed skills, or that are bigger/stronger, precisely because of their ability to "kill" the posture both under their throat and under their hip, either singly or in combination.
Let me know if that all makes sense to you.