There is no suction. The complete cooler line circuit is pressurized to maybe 25psi. The only suction in the trans is in the pump intake. To my knowledge, the way the inline flush machines work is with a bladder, where the expelled ATF displaces the same amount of new ATF, which is fed back into the return line of the cooling circuit. They do not rely on suction.
Really, a regular drain and fill is fine preventative maintenance. Most Toyotas use a screen as a filter and unless there is a problem in the trans, pulling the pan and washing the screen is not necessary. If there is a problem, dropping the pan and washing the screen won’t help, except maybe in the short term. It’s not like the old days when Dacron or paper element filters (which clogged easily) were used.
An aftermarket cooler is a good idea as well.
