Well you need to add as much weight as you need. The base line should be to be able to easily comfortably at 3m depth with an empty(!) Bcd with an almost empty tank like 40 bar left. That should be in (more or less... its recreational) neutral trim. The pressure differences are huge and so if you are upright that changes the buyoncy of the 7mm wetsuit for different body parts.
If you have more, you need to inflate your bcd more, so use more gas and have more resistence so have a higher force needed and that leads to increase of gas consume / average volume in lung.
Too much of lead, which you cannot release in emergemcy is dangerous! A 7mm looses quiet a lot of buyoncy between surface and 30m.
I believe its a combination of upright descent and finning, not exhaling enough so having a higher average lung volume, which can easily make 2 to 3kg of lead.
Also maybe at the beginning some air is trapped in the suit or bcd adding some more buyoncy. Sometimes you are not able to get all air out of the bcd if the geometry does not fit.
pyschologically it helps for me to breath out fully through the nose, equalizing the mask. Not sure if it has to do with my hood as well ...
Also to accept that descent is actually slow with a 7mm and you should breath easily is something. If you expect to descent fast, you may think you are not descanding at all, thinking it through -> chain reaction.
Some things you can try out and work on.
