It's good to hear that you tried to derive a word like 'sovereign'. By definition, being sovereign means ruling, which relates directly to "rel"

For example, "Kings are the sovereign leaders of kingdoms" would be "Kings are the rulling leaders of kingdoms" (Junne lost reltaas drogge do junaarre -- uses the present participle "-taas / -ing" ). Another way to convey sovereign is to reorder the sentence around to get "Kings rule in kingdoms" (Junne rel ko junaarre)

If you want to be more precise and say "Kings are the highest leaders of kingdoms" to differentiate them from Jarls, Dukes, what-have-you, then you can use "zok lot" (most great) -- "Junne los (fin) zok lot drogge do junaarre" (Kings are (the) most great leaders of kingdoms) or "Junne los zok lot do nust wo rel ko junaarre" (Kings are the most great of those who rule in kingdoms).