modern grower

I water the day after repotting IF it didn't damage the roots excessively. Root damage may increase changes of rot with cacti. Some people go extreme and wait weeks to rewater but a couple of days is enough if the potting mix was bone dry.

I live in southern brasil so we have temps as low as -2C for a couple of hours during winter. Summers in my region reach 35C MAX but rocks heat up and surpass 50C. From what I've observed is you just need to make sure the soil is aerated. The main downside of too much inorganic material is it drys out too quickly... I think 50/50 is a good compromise but depends on your local climate.

no I remove mine from high humidity after 2 months keep soil moist. they suffered a month long drought last summer due to a trip but survived. They were very skinny and dark. I don't reduce light when they get red or purple/brown. Keep them there and just week them moist and fertilize lightly if you want (I didn't) after a month or so they will grow just fine and quite fast. 50/50 granite chips(gravel) and clay based garden soil. I did have some losses but will end up with more than I know what to do with them. sowed 50 seeds total I think I have 20 now?

Here they are at 1 year. After the slow down from intense sunlight they will gave a growth rate increase.

Trichocereus can handle near constant moisture but watering once a week is enough... I personally do not reduce light even if they become very dark as the new growth will grow accustomed to it. I do not keep in high humidity for more than 2 months and bright sunlight (full auz sun might be too much) from there forward. Keep moist and water heavily each week. MY soil mix is 50/50 clay/granite(gravel) You can repot and any moment and as long as you are not too rough they can take a watering the following week. I repotted mine at 2-3 months they survived a month long drought in full summer sun at 4 months due to a trip. Remember to fertilize if you want but don't overdo it. A drought period is important doesn't need to be a full winter I usually do a month in spring and another in the summer and water all winter. here they are at 6 months.

They are growing etiolated due to not having enough light... Increase sunlight or bring the light closer and it'll grow normally from the apex.

Adapt or die... How many world events have happened and plants either remained dormant for years as seeds or just evolved to survive. Plants go extinct and sometimes reappear decades later.

Another experiment worth trying to induce flowering would be torching the cactus (not excessive) perhaps placing twigs all around it and burning it careful not to spread a fire. These experiments are quite limited for now and will be reattempted as time goes on using own root cacti and more mature trichocereus in the future.

If someone with a couple of extra mature trichocereus are willing to experiment... you could attempt girdling the trichocereus to varying degree to see if it'll induce flowering. Its used in fruit trees as a successful means to increase fruits and induce flowering at younger ages. These done with seedlings since its all I have currently.