If you are going to do a true water flush then use 3 times the size of the present container .
Your existing pots are maybe 10 liter pots ? Flush with 30 liters of water. Slowly.
Your problem will be drainage in your existing pots so go slowly.
Yes , MJ needs dry periods during veg. When your lower leafs start drooping , the plant is about 8 hrs from needing water. When you water , go slow . Water until you see it run out the bottom.
This entire process works much better in soft pots
PS. My sweet spot for dirt plants is 6.2 PH.
PPS. If ( they will ) your plants make it to flower , they will double in size during the stretch.
You might want to consider 7 gal pots ?