In sum (as shown in today's two previous posts), the primary meaning is apūrvakārya, but since this primary meaning cannot be learnt in the ordinary world, one has to get to the primary meaning trough a secondary one. One learns the secondary meaning in common usage, later one refines this understanding through Vedic study, otherwise the Veda would have no purpose.
At this point, one might wonder why is not the action to be done the one and only meaning of optative and similar suffixes. But this kriyākārya (action to be done, lit. "something to be done consisting in an action") cannot be the only meaning of an optative suffixe because otherwise sentences such as "the one who desires heaven should sacrifice" (svargakāmo yajeta) would not make sense since, as shown in the above chapters of Rāmānujācārya's Tantrarahasya (§9.-9.4.9), an action cannot be the means for the accomplishment of something desired as it expires much before the arousal of the sacrifice's result. Nor is it possible that the kriyākārya is the primary meaning and the apūrvakārya (the unpreceded duty) the secondary one, since one could not get to apūrvakārya through lakṣaṇā (secondary designation) starting from kriyā, since apūrvakārya cannot be known otherwise than through the Veda. Similarly, in Patañjali's Mahābhāṣya it is said that one learns the meaning of an apaśabda (incorrect word, such as goṇi instead of gauḥ) through lakṣaṇā, because it reminds one of the correct word. Interestingly enough, this means that the primary meaning does not necessarily precede chronologically the secondary one. In fact, one learns to speak Prakrit before Sanskrit and one learns the kriyākārya meaning of an optative suffixe prior to its apūrvakārya meaning.