I don't graft.
It is all in the limiting factors.
Light, nutrients, temp, water, pH, air etc.
I give them a lot of light and feed them but I do not actually give them excess fertilizer, I use a constant dilute solution of 20:20:20 at nearly every watering and I water often and heavily. The concentration of fertilizer is not high enough to harm microbial life despite being synthetic, which after all even organic nutrients produce the same cations as synthetics and can cause problems in excess. Plants respond better to a constant low concentration than they do intermittent strong concentrations of fertilizer with some exceptions due to buffering abilities of mediums with high ion exchange capacity which maintain a constant level of bioavailable nutrients even with periodic feedings. Nevertheless I don't provide an excess of nutrients which would cause salinity which is adverse to cacti.
So called hard grown plants aren't healthy nor strong, it's like trying to raise an animal and starving them to make them healthier and stronger. It doesn't work. Overfeeding and underfeeding both harm plants.
I recall seeing a collection in 2006 that wasn't growing much at all and I asked the owner what they used to feed the plants... they reported that they didn't feed the plants! Nitrogen is a limiting factor for Trichocereus, just like it is for Opuntia!