One thing that has not been mentioned and should have. Are your plants photo-period or auto-flower?
I grow in a natural or mineral soil mix and I have kept plants in a vegetating stage in one gallon pots for several months while waiting for room in the flowering cabinet or tent. Then about two or threes weeks before one or more plants in the flowering area were coming out I would transplant some of the waiting plants into a larger pot and let them grow new roots.
Things that slow down the plants include lowering the temperatures and reducing the amount of light they get. It is easy to set the lights for 14 or 15 hours on and 9 to 10 off. Then to prevent any of them from even trying to flower I used a 'gas-lantern method or schedule' which meant that I had half of the lights come on for an hour during the middle of the 9 or 10 hours the lights were off. This was enough to prevent the plants from flowering even though they were sexually mature. Definitely will need timers to pull this off.
If you are growing in a dirt or compost based soil then other things that will slow down the plant growth are reducing the amounts of water and fertilizers. Doing this is a bit trickier since it involves learning to notice when the plant is going to need water about 4 to 6 hours before it even starts to wilt. If the plant wilts too often or for too many hours it can damage the vascular system and some plants will never recover. Been there and done that.
And, all this time the plants are slowly growing. Reduce the fertilizing and they do OK if the soil is decent.
If you only have to wait about 5 to 6 weeks before putting them outside you might be OK even if growing is some of the mixes that contain a good amount of peat moss or coco coir.
Also, look up 'hardening off' plants that are going from inside the house to outside. These are methods of letting the plant get used to outdoor temperatures and being in sunlight.
Hopefully this gives you some ideas to think about.
