how to boot directly into the nrjql kernel without going into advanced options at boot/start?
Did you try
GRUB > Advanced option for OpenMandriva Lx > (list of available kernels) ?
yes, works that way but want to make it the only one, to boot directly into it without entering the menu
download from repo grub-customizer, it’s easy to use and you can define a default kernel
just an advice, it is not good to have a single kernel. if you have a problem you can find yourself in a very complicated situation.
Not tried recently, but as far as I can remember the entry you choose will become default also for next boot(s). Until you change again.