When you fill nearly half the area, you can expect it to fill in the rest after 12/12. It's hard to stick to it.. the general motivation is to grow it out more, but then you end up with a crowded canopy or having to prune stuff off to fit which is just wasted time and resources.
how you use a trellis will dictate when you put it out... if part of training, use it at the point the canopy is high enough that you can still properly irrigate without it in the way - this will conserve vertical height until everything gets into position. If for support only, you need it the last 8-10" of growth - maybe 3 weeks into flower?
Should only need to defoliate if you over-crowd the canopy. Removing an occasional leaf for a very good reason is fine, but never just whimsically remove a bunch of leaves as some sort of maintenance for the plant as it's not value-added. Removing leaves just makes a plany want to grow more leaves... could delay flower if you do it after 12/12 too.
If you want to see the type of varied stretch you can see, i have progressive pictures in my diary shot from same angle each week 28-70 (usually flip to flower 35-40). The strains show same progressive pictures. you can see from another angle too, but have to look per week.. check out flip to flower then 28+ days into flower and you can see all the gaps fill in. bottom trellis is ~5" squares and top is 7.5"