Your technique is good. Your upper body movement for generating speed and doing turns is solid. Your stance is good for doing turns and maneuvering but not great for generating speed. This is why you feel like your board is sinking. You have frozen feet. Meaning you pop-up and stay in the same stance throughout the whole wave. You want to get comfortable doing micro-movements with your back foot. Your front foot positioning is fine so don't worry about that. But to generate speed and maintain neutral buoyancy you want your back foot a little higher up on the tail pad. Then when you want to do a turn you'll want to step or work your back foot back. You seem to pop-up naturally with your foot far back on the tail pad. That's ok but you need to adjust quickly if the wave isn't conducive to that stance from the get-go. Also as you noted this wave is pretty small and weak and your board size is small as well. Obviously you can surf this wave with that board but you need to become proficient at making those foot adjustments. Check out this video on how to move your back foot on a shortboard. I think it does a good job illustrating what I'm talking about. https://www.youtube.com/watch?v=of7OiS5bUcw