With a bit of testing and from the comments of people helping, I have come to a conclusion.

#define T1ms 16000    // assumes using 16 MHz PIOSC (default setting for clock source)

That preprocessor directive is wrong due to not using a L293D and utilizing another type of motor driver specific to stepper motors called a DM332T driver from stepperonline-omc.

Now, if I define T1ms to 400 unlike in the example previously listed for the internal clock frequency, I can move my stepper in one direction in a faster RPM. So something like this:

#define T1ms 400

Or...if I was risky, I could test with:

#define T1ms 1 // this is if I would like 400 RPM with the current driver config

See, the driver has an internal couple of dipswitch settings that can be altered on the outside of the driver. This dipswitch setting appliance on the outside of the driver allows for faster RPM or more steps per RPM.

I have been reading theory recently and learning about how to control the STEP of the stepper motors and direction of the stepper motors too. I have been reading from here: https://www.orientalmotor.com/stepper-motors/technology/stepper-motor-basics.html

With the preprocessor directive of T1ms set to 1, I would need to fasten my motor to something heavy so not to throw safety in the wind. This way, the motor will not become disconnected from its source or location. I think with my questioning, this is the answer I was looking to attain.

Q: How can I make the motor move faster than what the internal clock allows?

and...

A: Use a driver with dipswitches and allow the driver to account for driving.