Also, I get that fear, but I do think it's quite foolproof is done properly (hyperventilation, proper technique with an asphyxiating gas, depth of 33+ feet)
Drowning at higher depths should increase reliability of CTB, but I suspect that it may be worse in terms of discomfort. If you perform gas asphyxiation before submerging and then dive deep, the external pressure will grow and the pressure of the gases in the lungs will grow too. Increased partial pressure of residual O2 may provoke waking up, that is an undesirable outcome.
In case of using an easily liquefiable gas such as n-butane, the situation may be even more funny. If you look at n-butane's pressure-temperature chart (
https://i.sanctioned-suicide.net/images/2025/02/220642_R_600_pT_EN.pdf), you can notice that the pressure of 3.5 atm (which we have at approximately 26 meters under the surface) is sufficient for turning gaseous n-butane into the liquid form at 37 °C or below. After this transition, the gaseous form will no longer be able to balance the external pressure like normal air (the fluid won't do this either because of a significantly higher density), so the lungs will collapse and you could be forced to inhale water to compensate this effect.
In order to reduce the risk of waking up at high depths, you'd need to carry out gas asphyxiation at the very bottom somehow. The chosen gas must not be liquefiable under the expected conditions, and if it's released into mouth, it shouldn't come straight from a vessel where it's present under a very high pressure.
For example, if you release N2O from an 8g charger into a cream dispenser whose volume is 0.5 liters, the resulting pressure inside will be approximately 8 atm added to the initial pressure there. Releasing such a highly pressurized gas into the throat may be somewhat unpleasant. When you asphyxiate yourself before submerging, you can use a latex balloon to make the pressure of N2O maximally comfortable for inhaling, but this is not an option when you're somewhere deep underwater. The issue can be mitigated by using a larger dispenser (with a volume of 1 liter or more) and/or some hose attached to its output.