5" RA8875 Display not working w/ Teensy 4.1

As KurtE mentioned, 9V batteries have very little current capability of < 100mA. Your display needs about 280mA not to mention the Teensy which would like about 100mA all by itself. Find yourself an AC adapter of 7V to 12V to plug into that breadboard power supply. Until you fix the power, nothing good is going to happen.

I also have a setup with the very similar 7" BuyDisplay LCD and it works fine with the RA8875 library that comes with TeensyDuino. I am running it on SPI1 like kd5rxt-mark, but that shouldn't matter.

You also don't need to be fooling around with Reset. You just tell it which Teensy pin it is connected to in the constructor and the library takes care of the rest.
Note: I would guess that this won't work with a simple 9v battery like that. Let alone usually the wires that connect them probably don't carry very much current. But I could be proven wrong...
I hope it's a supply problem, I'll make a 12 to 5V supply today to test with a proper power supply.
Let's see if this resolves the issues