Here "rich" is a nominal adjective. It can be stated as an adjective. Alternate translation: "Tell those who are rich" (See: [[rc://en/ta/man/translate/figs-nominaladj]])
Paul tells rich people not to confidently expect that their riches will always provide what they need. Alternate tranlation: "not to confidently expect that they will always have what they need, because it is uncertain how long they may have their many possession"
Paul tell rich people that they should confidently expect God to do what he has promised to do. Alternate translation: "they should confidently expect that God will do what he has promised to do" or "they should fully trust in God"
"all the things that will make us truly happy." The reference here may include physical objects, but it more likely refers to states like love, joy, and peace that people try to obtain by means of physical objects.