Manjaro receives the kernel directly from https://www.kernel.org/ so version 5.4 is the latest LTS Kernel. As long as you are on the 5.4 Kernel (doesn’t matter which version number: 5.4.69 or 5.4.56), you will be always able to use the 340xx driver.
Please read this here:
There is a patch for 5.6, 5.7, but on 5.8 there will be no patch for this specific version of the driver.