But I would have thought that if there was widespread 'central hypothyroidism', someone would have twigged by now, since that form does show up if you do a full panel of hormone tests

Which tests? I am not aware of any simple blood test that measures the endpoint of thyroid activity on metabolic rate (except, arguably, cholesterol levels), rather than just the state of the T4->TRH->TSH->T4 feedback loop.

mostly T4 with a bit of extra T3', but no-one has particularly clear ideas on what works and what doesn't or why

The challenge with T3 is it has a very short half-life, one would need to take very small doses impracticably often to achieve stable levels. Taking mostly T4 with a bit of T3 helps compensate for the reduction in T3 production due to feedback without the problems caused by trying to obtain nearly all T3 directly from a supplement.

Thanks for the reference to Ray Peat, I hadn't heard of him before. Can you link to the best expression of his thoughts?

His own essays at raypeat.com are the only accurate source, but can be challenging to read. Most of the summaries you will find online don't do him justice.