Yes, of course Waite was aware of earlier decks that had fully illustrated pips. He was actually quite disdainful of them.
"There in was a period, however, when the numbered cards were also pictures, but such devices were sporadic inventions of particular artists and were either conventional designs of the typical or allegorical kind, distinct from what is understood by symbolism, or they were illustrations—shall we say?—of manners, customs and periods. They were, in a word, adornments, and as such they did nothing to raise the significance of the Lesser Arcana to the plane of the Trumps Major; moreover, such variations are exceedingly few." Waite - The Pictorial Key to the Tarot
He goes on discussing the illustration of the minors, but the wording becomes more challenging and it is difficult to follow, often seeming more interested in insulting those that have gone before than trying to explain his own motivations. Still we are given the following:
"In the rectified Tarot which illustrates the present handbook, all numbered cards of the Lesser Arcana—the Aces only excepted—are furnished with figures or pictures to illustrate-but without exhausting—the divinatory meanings attached thereto."
. . .
"The mere numerical powers and bare words of the meanings are insufficient by themselves; but the pictures are like doors which open into unexpected chambers, or like a turn in the open road with a wide prospect beyond."
Which does seem to say that his goal was to convey the meaning fully and that cards are all illustrated because words, and numbers, alone are insufficient to that task.
One of the practices of the Golden Dawn was to meditate upon the illustrations of the majors to help the seeker more fully understand the meanings behind the cards; to use the card as a door to another world where these meanings were manifest. It seems the minors were to be illustrated in order to enable similar work to be done with them. And further that Waite considered this to be very different from the purpose of the illustrations accompanying the few decks that had previously been made with fully illustrated minors.