It is very rare the free driver doesn’t work with old cards - it must be very special.
The usual advise is
Use the free driver option in the installer - this will set the system to use the nouveau driver.
If it is completely impossible you can edit the kernel commandline - perhaps verify what kernel options your current install is using
cat /proc/cmdline
At the very least - you should be able to get to a desktop - then use the guide to build the old driver from the repo you mention or using the manjaro gitlab repo.
You should watch out for your kernel version according to this