"We could have had what we wanted, if we 'had' decided with ...." Don't forget the auxiliary verb (had) when constructing the past perfect simple tense. For the end of the sentence, I'm not sure exactly what you're trying to say when you use the word "intention". If you are trying to say that - had you been more sure of your decision you would have got what you wanted, then you could use the word "conviction". "We could have had what we wanted, if we had decided with conviction."

We could have had what we wanted if we had just tried/decided

"We could have had what we wanted...." - this is OK "... if we had just decided..." - this is more than simply following the rules for third conditional. You need to present the information clearly and in order. Use past perfect. "...properly." - Your phrase sounds very awkward and technical. Keep it simple. I would naturally say the phrase as, "We could have had what we wanted if we had just made up our minds."