Yes, I agree that a factory reset should be the last-ditch curative measure before sending the device for repair. I imagine that’s the first thing they’ll do with it anyway.
As I see it, a factory reset can be performed for either or both of two reasons:
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To distinguish between a HW and a SW problem, since we know that the HW is reputedly functional when running in initial conditions.
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To save time hunting for and solving a SW problem. But in that case, starting in Safe Mode should be attempted first (as suggested above by @anon9989719), as this will have, temporarily, a similar effect, as far as apps are concerned - though not all settings. If starting in Safe Mode makes the problem go away, then one can choose between either:
- a factory reset (preceded by backup and copious notes on apps and settings) followed by progressive reinstallation of the apps;
or - progressively removing the installed apps and / or returning to their default settings, which can take a long time.
So a lot depends how close the phone’s current configuration is to factory settings.
It’s important, when asking if a factory reset has been done, to remind users that this will wipe all their apps and data. The danger is, that some people may take the question for a suggestion… 
Sorry for this hot air, just thinking aloud really 