Hi Houston,
First, sorry for my answer but we can’t really handle site maintenance including bug fixes of existing customization. Please remember that customization added and given is only good for the time it’s given, especially for the active version at that time. Any maintenance and fixes are the responsibility of the site owners. This issue was an incompatibility between your custom codes and the latest version.
Procedures are given by Ruenel so I just repeated that the real issue is the child theme since it’s still the same. I just assumed there is a lot to fix so I recommend a developer that should able to help you since it’s still not fixed (but it’s very clear the cause is the child theme).
And we always recommend doing the updates to a staging version of the live site ( a clone of your main site ). So any issue could be fixed before pushing everything live. It’s common practice to any software maintenance.
Based on the discussion, the last issue was Ethos right? And to what version it was restored? I still see it with the latest versions, or are you just referring to the child theme?

But I checked and tested the Ethos stack and it works okay. May I see a video recording of the issue happening when you switch to Ethos?
Thanks!