The total cost of this device seems to be around 90€, while the cheapest QRP I found on Aliexpress is 100€. This difference might seem insignificant, but I bet that you have at least 70% of the supplies needed somewhere in your shack. The screens, buttons and leds are absolutely replaceable, but you'll need to tweak the code a it.

As of right now, the planned device will be composed of:

1. Raspberry Pi Zero

To take input from a potentiometer(encoder) in order to shift the frequency that we want to TX/RX.
To control the RTL-SDR stick and pipe the audio to the speaker, through the USB audio card.
To stream audio from the microphone, and pipe it to rpitx(this project is really cool)¹ which then will get amplified using the amp.

_{1. Please do read about this project, as it is vital stepping stone for this one.}

2. Arduino TM4638 board

Using this library.
To spare us of the pain of wiring an LCD.
View the current frequency we're at.
To let us change the bands, using the 8 buttons on the bottom.
Indicate states of the device using the LEDs.

Wiring

Sorry in advance for the trashy diagram, but this is my first ever attempt at wiring diagrams. The rtl-sdr and the audio card need to be put in the usb port, using a hub of some sort.