An unusual approach, but let's stipulate to that and move on.
It's likely that support for High Sierra will end when Big Sur is released, and that could be as soon as early October - you know, a couple of weeks from now. Also I didn't say that you "should" be using Catalina (though you probably should), just that the hardware should be capable of running it; I was wondering why you were troubleshooting using an old operating system when a current one is readily and freely available. There can be good reasons for sticking with High Sierra (mostly to do with support for 32-bit applications) and that could be relevant to your, or your client's, problem.
You can probably find "experts" to agree on anything. My experience, and that of my clients, is that Catalina is perfectly stable and runs well on any supported hardware. If this were your own computer (or you had your client's full confidence and a good backup) then I'd recommend upgrading to Catalina and retesting; clearly that's not an option you'd be prepared to consider, so we'll leave it at that.
But thank you for the additional information anyway.