I don't know what advice there is to give. I don't think it has anything to do with height. I also have a DocJohnson - Dik Rambone 2. There is a muscle that holds up the sigmoid that is somewhat under your conscious control. Also, please post your height as well. I can think of no practical reason for wanting to do this.
I've seen videos of people able to take the whole " without any issues
OP: Deep Anal Penetration
I'm able to take roughly 6" of it roughly the full length of the rectumbut I cannot get it past the 1st bend. I've seen videos of people able to take the whole " without any issues Why do you want it in your actual colon, if you don't mind me asking? Any less often, you'll need to start over. Also, please post your height as well. If you are after bragging rights, you will pay in the long term.