Thanks so much for all the replies, guys. I solved my problem, and wanted to come back and share some info about the process I went through in hopes I can help others like me that felt like the 1.5" option was a bit too high.
When I installed the 1.5" pedal spacer from ASD, I could see that, as advertised, the gas pedal was about even with the brake pedal. Sounds good, right? Well, the problem is that with the gas pedal shifted that much closer to the driver, it is immediately apparent that the gas pedal is very responsive, even with very little throttle input. On the other hand, this also makes it apparent that the brake pedal actually has a fair amount of travel, and is not very responsive with minimal input when driving around at normal speeds, especially compared to the gas pedal at the same height. As a result, after installing the spacer, you get an exaggerated sensation that you have to push the brake down way farther than the gas pedal all the time. This is the exact opposite sensation from the stock setup, where it feels like you have to push the accelerator down way farther than the brake. Rather than these two extremes, what I wanted was for the gas and brake pedal to feel comparably aligned and in-sync with one another, much like other sports cars I've driven in my life. If I had to guess, I think that was the intention of ASD and other OEMs that produce pedal spacers at 1.5", but because of the travel of the brake pedal on the Focus ST, setting the height of the gas pedal even with the brake pedal does not, at least for me, seem to provide that result. Before I move on to my solution, I will say that I've seen in other threads that other drivers originally thought the 1.5" spacer options were too tall, but after a couple weeks got used to it. I experienced this same thing; my muscle memory adjusted, and it didn't feel quite so bizarre anymore after a few days. But even after I had gotten used to it, it still didn't feel quite right. I realize, of course, that these things tend to be subjective, and what feels "off" for me might be perfect for someone else. My ASD spacer was a very high-quality piece, so this is certainly not a knock on them. The problem is that the brake pedal travel on the ST makes things difficult.
Anyhow, in search of a more "moderate" pedal spacer, I found the following options and heights:
Active Shift Designs - 1.5"
GarageLine - 38 mm (~1.5") or 55 mm (2.16")
Breedt - Thick (~1.5") or Thin (~1.2")
Your Little CNC Shop Focus RS Pedal Spacer - 3/8" (Out of stock and not available for purchase the past few weeks)
MetalFabByEric Focus ST/RS Pedal Spacer - 1/4"
At first glance, none of these options were what I thought I wanted. To me, the gas pedal with a 1.5" spacer felt so much more responsive than the brake that I thought a 0.75" - 1" would be best, but it seems that no one makes them in this height. However, I decided I would take a chance on one or two of the options above to see if one of them felt right. I first decided to reach out to Breedt. As I mentioned on my first post, their site only listed the thick option in the drop down menu, so I thought they no longer offered the thin option. However, after contacting them via their website, I exchanged e-mails with someone named Craig who was super helpful. He promised they would update the site so that the thin option was listed again, which they did immediately. To me, the idea of going from a 1.5" spacer to around a 1.2", when I measured that out using a ruler, seemed like such a small change that it would be almost pointless. However, Craig mentioned that he used it himself, and that it was a difference you could feel. With that in mind, I decided to give it a shot.
Once the product arrived, I got everything installed and set the spacer up using the option that kept the pedal at the stock distance from the brake, but ~1.2" closer to the driver (the top-right mounting option). Much to my surprise, this option was PERFECT. The thin spacer sets the gas pedal slightly behind the brake pedal, but because of how far the brake pedal travels compared to the gas, this slight recession is exactly what is needed to make the two pedals feel like they have about the same amount of travel. Further, this sets the gas pedal up to be about even with the brake when applying normal brake pressure, meaning that heel-toe shifting is perfect around town for spirited driving. For me, this is the perfect combo: the gas pedal is still much closer than stock, making it much easier to get the pedal to the metal, but also seems to put all three pedals in-sync in terms of travel. For those of you searching for something a little thinner than the 1.5" spacer, don't hesitate to give the thin Breedt spacer a try. Thanks, Breedt!