I don't recall the exact model of the box, but I recently re-visited an installation with an X1 box that came supplied with an XR2 remote. This installation had been working fine for about two years. I use Global Caché for the IR emitter. Suddenly, about a month ago, virtually all of the codes stopped working or became intermittent. I had to drop the carrier frequency a bit to restore operation. I think that these boxes are extremely sensitive to the carrier frequency and that Comcast updated the firmware and tightened up the tolerance to carrier frequency. Currently I'm using a carrier frequency of 38,000. Note that there will be truncation and rounding errors along the way as various boxes and programs recalculate and pass the data between them. 38,000 works well in this installation, you may be better off with a slightly different carrier frequency.

I've been using delays of about 200ms between digits, knowing that this is a little tight. I try to walk along the line between sluggish and predictable response. Delays between other commands need to be a little longer.

Longer delays are not a bad idea. I'm quick with button pushing. When I'm using the native XR2 remote I habitually overrun these boxes, but I have noticed that later X1 firmware/boxes tolerate my lightning fingers a little better. The minimum delay is slightly variable because the later firmware versions execute a progressive alpha search as more digits are entered and one should wait until data reflecting the currently entered digits is posted to the screen before sending the next digit. Unfortunately, your program cannot know exactly when posting is complete and it is possible (I'm not sure about this) that data is being fetched from a remote server after each digit is entered (implying slightly variable timing). [Actually, the latest firmware seems to multi-task a little better and I can overlap sending the next digit and waiting for the response -- slightly -- you'll need to experiment to find out what works for you]

By the way, I queue the digits entered and execute the whole pack in a metered sequence. This results in the most reliable operation. I make sure that all of my channel commands are four digits or follow shorter selections with an "OK" command. This will start execution as early as possible. I'll follow up with "Exit" in an effort to suppress that pop-up. You'll need to tweak the delay between "OK" and "Exit".

FWIW: This is the wrong area for a post such as this. You may not get much of a response.