raspi-config is just a shell script with minimal dependencies. Grab a copy from here:
https://github.com/RPi-Distro/raspi-config/blob/master/raspi-config
For most things including this it should work fine as Raspbian is in fact nearly identical to Debian. There may be a few thing that require the userland tools but this is not one of them.
All it appears to to do is add enable_uart=1 to /boot/config.txt, and, if you want the serial line console disabled (you probably do if you want to do anything other than login or observe system messages over UART), it removes console=serial0 or console=ttyAMA0 /boot/cmdline.txt. The best bet is to change that to console=tty1, which will put system messages on a virtual console (the one you see on the screen if you boot without a GUI).
The official config.txt documentation regarding the UART is here: https://github.com/raspberrypi/documentation/blob/master/configuration/uart.md
unrar-nonfreeorperf, so Debian is not entirely useless as you suggest. – Dmitry Grigoryev Aug 17 '21 at 12:55