Hewwo Plise!
I am now reaching nearly 29,000 cargospace.
I see in CTlab you can make those +1000 capacity to all resources with the cargo space expanders.
So when should I stop using them to gain cargospace, but instead use them in CTLab?

I sell artifacts fairly regularly, and I don't carry a lot of minerals around on a daily basis, my 75k cargo bay still stays 2/3rds full.
So, there is no set point at which you should "stop" cargo. Now in terms of when to start making those CTLab buildings, I would say the sooner the better. You can only add one per planet. Over long enough a time, you will even have those maxed out. Then you are back to either adding cargo or just storing mass pods until you get another planet and add the building.

As soon as you want to.

when u dont need the space make vaults till u need the space again.

Even if you put all of you mass storage pods into vaults, you should have a slight surplus of tesseract containers to keep expanding your cargo capacity.

the minute you cap and lose some of your production.. ive got 45K cargo, perpetually half full, and 162K extra arti storage and now capping in just over 9 hours, just remember that you can only put one of them on each planet, so unless you put one on every planet (which is unlikely) they arent really going to add an enormous amount.

I maintain a 100:1 cargo space to rank ratio (115,200 cargo at rank 1152). Trying to keep it at only 75% full to still be able to hold minerals when drawing down or trading. Rest of msp go to vaults or are kept on hand.

I keep 700 of most artifacts on hand in case Dan springs a mission on us that uses them. Ideally I keep 10k cargo empty to hold mineral shipments, so if I'm under 10k free I add space. (Currently have 60k total.). Otherwise I just make vaults like mad. I have em on about half my planets.