Is it safe to fit a rear mounted (boot) cycle rack? The only specific Kamiq info I've seen said no but that was from a company called 'The Roof Box Company' and so I believe they might be biased toward roof bars😄

Make contact with them.  Their name certainly does not reflect that they also do rear cycle carriers and tow ball mounted as well as hatch mounted.   Quality items readily available, but then not all modern rear hatches are suitable to hang a rack and bikes onto.