This is the default OS that is provided by the Raspberry Pi foundation and comes preinstalled on the micro SD card that the Hackberry ships with.

Amend the config.txt with the following line:

dtoverlay=vc4-kms-dpi-hyperpixel4sq

If you mount the micro SD card on a computer, it will be located on the bootfs partition under /firmware/config.txt.
If you want to edit the file on a running Raspberry Pi (either through SSH or by attaching an external display via HDMI), it will be located under /boot/firmware/config.txt.

Screen blanking should be disabled by default, but it is never a bad idea to manually disable it by running sudo raspi-config and navigating to Display Options > Screen Blanking.

The following image is a clean install of Raspberry Pi OS. It comes with a configured config.txt to enable the display. Furthermore, it comes with Pi-Apps and Alacritty preinstalled. It also includes the battery voltage script.
rpios-base-compressed.img.xz
rpios-base.checksum.txt
Screen blanking should be disabled by default, but it is never a bad idea to manually disable it by running sudo raspi-config and navigating to Display Options > Screen Blanking.

If you fail to get an external display working, make sure you're using Wayland.
To do so, run sudo raspi-config and go to Advanced Options > Wayland and choose Wayfire, then reboot. The display should now be recognized and work automatically.
Use the Screen Configuration application to configure the orientation and arrangement.

It is recommended to install SDR++ by compiling it from source. Do the following:

# Install dependencies
sudo apt install git cmake libfftw3-dev libglfw3-dev libvolk2-dev libglew-dev portaudio19-dev libzstd1 libzstd-dev libairspyhf-dev libiio-dev libad9361-dev libairspy-dev librtlsdr-dev libcodec2-dev librtaudio-dev libhackrf-dev

# Download the code
cd ~/Downloads
git clone https://github.com/AlexandreRouma/SDRPlusPlus.git

# Build
cd SDRPlusPlus
mkdir build
cd build
cmake ..
make -j4

# Install
sudo make install

You can now run SDR++ from the applications' menu or from the terminal with ''sdrpp'.
Make sure you're connected to the bluetooth speakers or any other audio device. In SDR++, scroll down to the Sinks tab, make sure the right audio device is selected, and set the bitrate to 44100.