Hi John,
Unfortunately, it's all too easy to damage those delicate flex cables an the connections to the chips mounted on them, especially with the amount of glue and length of cable used on this iPad.
At this point, realistically all you can do is to recheck the connector where the home button plugs into the motherboard. Unplug it and clean the flex cable with some 90% or higher isopropyl (rubbing) alcohol then resecure it and retest.
[image|3290952]
If that doesn't do the job, the next step is to carefully examine the home button's flex cable for any kind of tears, scratches or damage. With some fair-to-middling solder skills it's possible to replace the length of flex cable most likely to get damaged as shown in this video.
[link|https://www.youtube.com/watch?v=Gwa79hu_4iU|Apple iPad home button torn Flex / cable repair - YouTube|new_window=true]
This is showing the procedure on an iPad 7, but as nearly as I can tell the iPad 8 uses a very similar flex cable so the same operation should work on it as well. Outside of that, the only other possibility would be to locate the diode mode readings for the home button connector and test them, but unfortunately I don't know if they even exist. I did find some for an iPhone 7 home button I was trying to repair, so it's not impossible, but I don't know where you might find them. Here's some connector information for the motherboard I ran across.
[link|https://drive.google.com/file/d/1IxWSUw3_HaBQCbQir1FQGls9tsstlCVC/view|J3800.rar - Google Drive|new_window=true]