When David decided to begin trying to 'engineer' the 'perfect organism'...

David has some drawings about the planet 4 inhabitants in his workshop, it means, he could examine the structure of their body. There's a gap in the crossing video, so maybe you are right.

But I think, he bombed the planet 4 when they arrived. It's just one city, maYbe there were more. Or there were some inhabitants, who survived the bombation. So he had hosts in this case too.

The particular pathogen he used clearly did not destroy 'every single potential host he could use' when he bombed the city. The specimens in his lab indicate he had been experimenting on the full range of non-botanical life on the planet and engineering his own creations for years. He even had an intact Engineer on display, skinned.

At some point he did wipe out all of the animal life though, or at least in his proximity he did. That is why there were no birds, insects, or anything other than plants to be seen when the Covenant crew arrived.

@Kethol: It would have taken him decades to experiment on every form of non-botanical life such as birds and insects, and they proliferate at a much faster rate than mammals.  So I wonder at what point he decided they all needed to go?

I doubt he decided they all needed to go, and he never said he experimented on 'every form' of non-botanical life. That would be impossible. There were only several dozens different animals shown in his lab.

I don't think he could control the spread of the various pathogens he released, so when all of the animals were gone, he could not continue.

It would be cool if David was merely acting on special programming imprinted by Weyland that would specifically lead to the Xenomorph, like W-Y knew all along that the Xenovirus would (naturally) produce a Xenomorph if exposed to the correct set of circumstances (which would need someone to do, ergo - programmable androids). "Shock troops of the genetic assault".