I'd suggest diversifying your sources of passages to find some that are more representative. KA passages tend to be very short and information-driven (i.e., with a limited focus on experimental findings), and sometimes include information that is beyond scope. For example, just to refresh my memory before writing this reply, I randomly clicked through a few B/B passages, and came across one that required you to know that the bladder is characterized by having transitional epithelium. In reality, here's literally all the AAMC has to say about the bladder in their description of what's on the MCAT: "Storage and elimination: ureter, bladder, urethra." There's also a single mention of "epithelial cells" on the AAMC content outline, with no additional details. So, in other words, the KA material is directly testing you on material that is really beyond the scope of the MCAT—for the MCAT, you need to know what the bladder is in general terms, and what epithelial cells are in general, but you don't need to know the precise type of epithelium in the bladder.
So, the reaction of "there always seem to be content gaps" with the KA academy material is very much on point, because there's not always a great match between what they expect you to know and what you actually need to know. Focusing on these marginal details is not an effective time investment.
The other thing about the KA passages is that they tend to overrepresent direct factual recall questions, which only account for ~35% of science questions on the actual exam, and underrepresent problem-solving/reasoning questions (~45%) and reasoning about experimental design/data (~10% each). (To be fair, they do have some of those questions...just that in my opinion, the balance doesn't feel quite right.) Plus, as a secondary point, the interface isn't representative at all. So...KA passages may have a role if you just want to kind of focus on science topics you have trouble with, but they shouldn't be such a major focus. (As a minor note in that regard -- the SDN study plan, for example, includes KA videos, but not KA passages).
In terms of other resources to consult, in addition to UWorld, I've had very good experiences working w/ AAMC materials fairly early on in the study plan—just not the FLs, so you don't feel like you "burn" a practice exam. For example, the Online Official Guide questions are an excellent set of medium-difficulty passages that give you a good feel for how the AAMC likes to test science content, which can help you focus your science studying accordingly.
