Update: kind of found a solution thanks to a comment on a YouTube video. It suggested to tighten up the belt for the Y-axis (bed axis), and this solved the problem at 90%. The test circles I printed are no longer lemon-shaped and look good enough to me (even though there still is the same effect, but minimized).
Here is the video: youtu.be/AF0wNn1oUiM and the comment is from Gimpdiggity:
I adjusted the x-axis today and did a quick test print of a circular item. A tiny better, but still noticeably not a circle. Figuring I was going to probably end up exchanging this printer for another, I figured what the heck and tore into it. It's actually not difficult to get down to the y-axis belt. The four screws that hold the glass bed in place, then three screws that are visible under the glass are all that's necessary to get to the belt enclosure...then that enclosure just snaps off . Once I was in there, I noticed that the y-axis belt was VERY loose compared to the x-axis belt. I couldn't really see back in the back (I didn't have the case disassembled) so I grabbed my phone and took a picture of the spring assembly that keeps the y-axis belt tensioned. It has a screw (same T10 as all the others) behind it that you can't see. A quick half turn to loosen that screw and the entire assembly snapped back a good 1/8th of an inch or so...that screw had the whole assembly so tight that the spring that keeps tension wasn't even actually doing anything other than remaining compressed all the time. I loosened it a bit more and let it settle, then I tightened that screw back down.
A second test print resulted in about as close to perfect as a circle can get!!!! Awesome!!!
I'm running a second test print of a more substantial object right now to make sure, but I think I've got the problem sorted, and I won't have to be sending this one anywhere after all!!!!
This gave me a great improvement in the shape of the circles, yet not perfect. I think if I tighten them even more maybe it'll be good.