EDIT: The solution of editing etc/default/grub did not work for me.
Newer kernels boot, but for they have other problems that can’t be solved, so I can’t use them.
Yes, with nothing … ie you should delete it as it probably caused an error. You just needed to add the SYSTEMD_CGROUP... to the existing content, if any.
Newer kernels give me terrible sound issues, that’s why I am stuck with 5.4. (I have posted here about the audio issues but there was no solution found.)
Friday I willl try to do it how @Denis_Pom suggested.
@MeuKey for me what worked for now is a Timeshift restore and newer Kernels.