There are two other "tricks" I've read about, but not tried myself, that you can use to try and get better pressure when using a large stamp.
First you could try placing a flat, sturdy piece of plywood on the floor, put your card front down, put down the inked stamp then STAND ON IT.
Another option is to stamp on a surface that is the right height for you to be able to "sit" on the stamp to apply all over even pressure.
I have not tried these methods myself but others have had success with this if you feel you've tried all other solutions and want to be able to apply more all over even pressure in the hopes of getting a more solid image. Just saying..