Welcome to OC dot org. The correct choice of a retention holster is kind of like asking what kind of firearm should one carry. So many variables. It is my understanding a regular holster with slip in and slip out only is a level 1 yours would be a level 2.
The easer the thumb breaks the easier for someone else to undo it. The more secure the the retention system the more time is takes to get it out in a hurry. How much practicing are you willing to do? Your thumb break holster might be fine depending on how easy it is for a bystander to undo it.
Not sure what to say about the knife thing. If it is time to use lethal force why go for the knife? One could find themselves at a gunfight armed with the knife instead of the firearm. Just my opinion. If you want, a CCW one place will do it free. I am on a mission not to ever spend a dime in the store however, the gun store on Trop will do the training for free you have to use and buy their ammo. It is my understanding they never spent the money for a ventilation system to allow them to shoot lead. If true a good excuse to make an additional profit on the non-lead ammo you are required to buy.