What is Wallace's true motivation?

As for the nomenclature: they are Nexus 9. From http://roadto2049.bladerunnermovie.com/year/2036 :

Wallace reintroduces a new line of "perfected" Replicants - The Nexus 9.

As for emotional connections between replicants: It is not said they don't feel emotions, only that they can't lie and have to obey. Nexus 9 replicants are very capable to feeling emotions, as we see Luv crying at several points in the movie regarding her "kind" (when Wallace dares her to say another is "born", when Madam says K killed the child). The key is that they obey their masters above their feelings. So reproducing would not be difficult.

As for Wallace being a clone: there is no cloning technology mentioned in the Blade Runner movies; the "cloning" of Rachel is only from a replicant, which are designed in first place, and have "blueprints". Wallace corp. already has "some fragments" of information on her, as Luv states. Rachel's well preserved bones provide enough for creating another one almost identical, but without the key to her pregnancy, and other unspecified details (like her eye color). So we have no indication how hard it would be to make a replicant who is a clone of a human, specially before Nexus 9 technology, and before Wallace's company bought Tyrel's, an interim where no replicant was fabricated.

Also, while Luv hurts from the prospect of the newly created replicant being killed, Wallace only sees them as slaves ("disposable work force") and tools ("keys", "locks") for mankind. His motivations for mankind to survive by "crushing Earth to suit its needs" are clear at least since 2036.

Since he predates the Nexus 9, if he is a replicant, he had to be a Nexus 8, created circa 2020 by Tyrel Corp before it went bankrupt (if he was a "Nexus 7" as Deckard, he would not need him). This possibility would then imply that he fakes his frailty, since replicants are physically superior; his blindness might be a fabrication created as a cover for him removing his identifiable eye. There simply is not enough information to be certain one way or the other yet.

However, like with Deckard's conundrum and K's doubts about his own origin, these uncertainties are at the heart of the Blade Runner ethos, and how the origins of consciousness are secondary to one's actions and choices. And we know Wallace's are ambitious, to say the least, and opposite to all other replicants we've seem so far (who have all good aspirations towards their "race".).

The replicant he disposed of was created as a test to determine if he could successfully create a fertile replicant. At the level of serving the Earth-bound human population, he can certainly create enough for that purpose. Having that failed experiment around makes zero impact on that. He's talking about expanding the reach of humanity in the vast, limitless reaches of space. That's where he needs to ability to exponentially replicate the replicants. The failed experiment serves no usefulness in that regard, either, and if he's unable to get fertile creations, keeping this one around or not will not impact that in any way.

Plus he was frustrated with that failed experiment, probably thought of that replicant as a failure, and having it around would be a reminder of his failure, which is something high-ego people tend not to like.

Keep in mind that replicants are not the same as clones.