SO now, the late answer (wanted to make it right) to how to install openCV on the Raspberry Pi with FreeType font support.
The easiest way is to install freetype from the official repositories and compile harfbuzz from source. Compiling Freetype from source is also possible however, if a newer version is required. I used harfbuzz version 8.0.1 (but also a newer version can be used probably).
First install harfbuzz dependencies (and compilers) by:
sudo apt-get install cmake unzip meson pkg-config ragel gtk-doc-tools gcc g++ libfreetype6-dev libglib2.0-dev libcairo2-dev -y
Then get an official harfbuzz release from github (currently possible) by:
wget -O harfbuzz_**x_y_z**.zip https://github.com/harfbuzz/harfbuzz/archive/refs/tags/**x.y.z**.zip
Exchange x, y, and z with your desired version.
unzip harfbuzz_**x_y_z**.zip
cd ./harfbuzz-**x.y.z**/
meson setup build && meson test -Cbuild
sudo meson install -C build
cd ~
rm -rf ./harfbuzz-**x.y.z**/
rm harfbuzz_**x_y_z**.zip
Then install openCV with your desired modules AND the compile option -D WITH_FREETYPE=ON
A good guide can be found here Install OpenCV on Raspberry Pi - Q-engineering (qengineering.eu)