Is it permissible to pray for something unseen, such as asking for peace of mind when blessed with righteous offspring upon marriage, or upon succeeding in an academic subject?

Supplication (Du'a) is an act of worship and a weapon for the believer. One must adhere to its etiquette and have a good opinion of Allah, for Allah answers supplications in one of three ways: by granting the request immediately, by storing something better for the supplicant, or by averting an equivalent evil. The non-fulfillment of a request might be better for the supplicant due to their ignorance of the potential harm it contains. The servant should supplicate for the good of this world and the hereafter, and rest is part of this good. If one is hesitant about a matter, they should perform Istikhara (prayer for guidance). True and complete rest will be attained upon meeting Allah in Paradise.