I’ve read that a new fresh image install is needed to update to kernel 5.4
How can I do to do that, without losing all the softwares and their relative configs?
Best to my knowledge, there is no way of doing so because we changed the base image to Armbian. This is a different image compare to the one we used before. As well file system layout changed to a single ext4 partition.
Yes, sadly no direct upgrade possible 
. Our Buster => Bullseye upgrade guide, which recommends to flash Bullseye images freshly where applicable, covers some rough instructions about which directories to copy or look at for individual data migrations to a new system: https://dietpi.com/blog/?p=811#2.1-fresh-install
At least there is one good thing when doing a fresh installation: One learns to document custom changes to the system, with some knowledge writes script to apply these after initial setup, learns where data is stored which must be copied, etc. In any case, keep the old SD card/system drive for a while so you can copy anything back into the new system, when you recognise that something is missing.