You are starting to sink immediately because after you flood the main ballast tanks you have negative buoyancy as the negative tank is also filled. Once you are submerged you should empty the negative. As soon as possible, as it takes quite a bit of air once you get deeper. But not entirely empty it! To get properly neutrally buoyant at periscope depth you should leave 0.05 m^3 (or 1 halve tick) in the negative tank.
At deeper levels you may need to empty it totally and empty the main ballast tanks also a bit to become lighter and compensate for your more compressed volume. How much I don't know yet.
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