dev/Serial0/Serial1 error on Teensy 4.1

It has been awhile since I played with RPI 3 but I think it only had one real hardware uart and it defaulted to Bluetooth.

There was way to switch to having the pins there use it instead…

Also they may have debug monitor or the like that uses the port as well, which you may need or want to disable.

I believe I was running at a speed at least 1mbs
I can't explain why your Raspberry Pi is stuck at receiving only with 9600 baud. Maybe a question for Raspberry Pi's forum?

I can tell you the serial baud rates on Teensy 4.1 work very well, least above about 1144. When you configure Teensy 4.1 with Serial1.begin(115200), I can assure you it really is 115200 baud. Likewise for 1000000, Teensy 4.1 is able to use 1 Mbit/sec quite well.

I'm pretty sure the problem is with the Raspberry Pi side.
But it's reported to work at 9600 baud, and at 2400 baud the Raspberry Pi does receive garbage which looks like wrong baud rate.

Sure looks like one side is changing baud rate as instructed and the other side is stuck at 9600.
All my RPIs (from 0 to 4B) work without issues at 115200 via UART (pins 8 and 10 + GND).
So I would say it's not strictly a RPI_UART-related problem.
As I mentioned when I was using RPI3 on robot with Teensy connected to these pins, I would run into some interesting issues.
Including that the UART had lower priority than the Bluetooth, plus console might also try to use it...

Here is a webpage, that talks about it:
I believe the last time I played with it, the raspi-config had stuff to do most of the stuff in this.

It has been a few years since I played with it, but the program on the RPI I ran (C++) used the termios library to configure the port.
And I know that I ran it, with at least 1mbs, I believe I tried 2, maybe 2.5
On the Raspberry pi, the shell messages on the serial connection could be enabled. This need to be disable to use the serial port for your own application.

Use raspi-config. P6 to disable it.