Have a goal.... when you work backward from a target canopy, it makes every decision easier in this regard. You will avoid a lot of wasted effort and resources on growth that you will never use.
Do you get there more easily training and topping a base of 4 branches or 6? Easy to double them up with topping. 4-8-16 cola progression or 6-12-24 etc... Balance is good when possible. More difficult to maintain a level canopy if all primary brnaches don't follow a similar path. Still doable, of course. I'd prefer to have 10 vertical branches per plant, but i do 8 or 12 for this reason. Even 2 colas per sq foot will damn near max out yield and with some phenos is enough.
You should wait until 6-7 nodes before topping, so this is a good time to do it. Shoot for 2.5-3 colas per sq ft (without being too anal about it) and make a plan to get there. By having some discipline in regard to number of colas in canopy, you can avoid a lot of maintenance work -- defoliation can be avoided and automatically get excellent air flow and light penetration. More colas won't improve yield but will spread the bud mass out more making smaller buds and generally more work trimming them. fwiw my best results have been with 2.5 colas/sq ft, give or take.
Look into using a scrog. Bigger plants are best with 2 -- lowest level helps train them horizontally, and second level supports 2/3rds up the height of colas. I waited 3 years to use a scrog and it was a mistake. if you do sea of green method, don't need one for short sturdy plants... or with simple training may just want the support-level scrog -- protects you against the occasional floppy plant that can't hold its weight in flower. You will come across plants like that eventually.
future reference -- the pebbles at the bottom don't really do anything as long as it doesn't sit in its runoff, so you can avoid that hassle if you want, but you should have drainage amendment of some sort mixed into your medium. Doesn't save much time with 1 pot but with 8-12 you'll appreciate a simple process. easier to just get a re-usable pot elevator or use a drain table -- automated runoff handling with a pump/float switch is something you should definitely look to add to your setup. Garden trays are expensive, but diy roof panel drain tables are about 1/5th the price and keep pots well above any potential standing water.
'Light mix' is coco, so overall its coco and hummus? Not sure of water capacity of hummus (sure google has that answer), but coco needs about 33% of volume to be perlite for best aeration/drainage properties. A higher water capacity soil or sphagnum peat moss should have 50% of volume for same drainage/aeration properties. Any non-homogenous mix you can apply a weighted average for the correct amount to use. While you can precisely calculate a value, again, this is one of those things you don't have to be super anal about. Just make sure it is close enough.
There's a lot of magic projected onto coco. it's just a solid medium that is not optimal on its own in regard to drainage/aeration. it has 'better' properties as-is, but not optimal.
Not world ending, but definitely worth adjusting in future. Big chunky options impedes roots. vermiculute #3 (coarse) or perlite #2 are excellent options. I prefer vermiculite because it doesn't float to the top as easily and adds some plant-available silica. avoid huge wood chunks, large clay balls, extra coarse verm or perlite etc...