This guy fascs.

This is the correct answer. It is literally the point of the piece.

You are doing it right; it just isn't easy yet- you just currently need a little more air than that. Getting in the habit of slow breaths forces everything else. As you calm down and become more economica in your movements, your breath tidal volume will decrease, and so will the bob.

Also: deep breath does not mean "max" breath. The SPEED is the important part.

Everyone bobs a little at rest if they are maintaining a normal respiration cycle. You can limit the bob by lowering the size of thel breath as you stated above- this gets easier as you get more comfortable on less air, which is a function of both mindset and developing exonomy-of-motion and economy-of-thought experience. When you get well and truly bored on the safety line, you can get that heart bpm down below 60 and it gets easier.

THAT SAID:

We don't control buoyancy to a station keeping point except for pictures and interacting with structure. Most of the time, the ask is to "swim at about this depth" in which case you either take reasonable breaths and not exhale all the way to ascend, or take reasonable breaths and exhale everything to descend. I call it breathing "out of the top" or "out of the bottom" of your lungs.

Easiest way to learn in shallow water, btw, is putting your hands behind your back and doing "finup" planks with your nose to the sand. If your nose hits the sand, you goofed.

Final tip for dealing with going ,up-and-dowm on dives, adjust your angle-of-attack and head either a little bit down or a little bit up to counteract.

Lowkey hilarious.