Native USB with programming capability

Hi all,

I am trying to work out how can I build a USB to USB keyboard emulator.
The idea is this device will be in-between a USB control host like ( pc / RPi ) and act like a USB Keyboard to target host for remote management purposes.

I have seen examples with Pro Micro and Teensy and got them to work but to program it, I need to disconnect this device from target host and connect to control host to program it.

I read that devices could be programmed over SPI.

Is there any development board which has both native usb (for keyboard emulation) + serial over usb ( for controlling and programming ) that I could use?
If not, what would be the simplest and cheapest way to build something like this.

Thanks in advance.

Why?