The problem here is the notion of "the generic image of emptiness." Emptiness cannot have a generic image since emptiness is not a thing, like a vase and its blueness. Meditating on the concept, "this is empty" is also unworkable. Of course, when we are learning about emptiness, of course we have to come to confidence in emptiness rationally, through reasonings. But when it comes to applying confidence in meditation, it is the position of the Sakya and Nyingma schools that focusing on a concept of emptiness is not a correct vipaśyāna, and in fact can lead to rebirth in the formless realms.
The Gelugpa tradition disagrees that emptiness cannot have a generic image - it is the mere absence of inherent existence. We can have a generic image of a non-thing.
Meditating on emptiness doesn't lead to rebirth in the formless realms - attaining form realm concentrations with a mundane motivation creates unfluctuating throwing karma that leads to rebirth in the formless realms. Furthermore, because meditation on emptiness is with the motivation of conventional bodhichitta, it cannot be the cause of any kind of samsaric rebirth at all.
Correct vipaśyāna meditation on emptiness is resting the mind in an objectless equipoise discovered through exhausting all possible conceptual proliferation concerning entities in terms of all modes of their existence, as Śantideva notes. It is not resting the mind on the concept that results from conceptual analysis, rather, the mind that deconstructs even the notion., "this is ultimate" and rests in that state, free of proliferation.
That's not what Shantideva is saying at all. This is an alternative translation of the verse you quoted:
Eventually, when the true existence of things and the true existence of emptiness
No longer appear to the mind,
Since there is no other aspect of true existence,
The mind will abide in the resultant pacified state in which all conceptuality has ceased.
What Shantideva is saying is that when we reach the path of seeing ('eventually') we will experience emptiness directly, that is, non-conceptually but it is impossible to experience emptiness directly on the paths of accumulation and preparation. This direct realisation is the result of many meditations on the generic image of emptiness on the paths of accumulation and preparation. If it were, indeed, possible to meditate on emptiness without a generic image, then the paths of accumulation and preparation would also be paths of seeing - in fact, it would be pointless to talk about a path of seeing because you would have a direct realisation of emptiness from the very beginning of your meditation.