1500km would indeed have been much too short distance; 5000 is adequate.
You've gone by the book - therefore there is obviously a problem that 7-rocks needs to look into for you.

The planing of the pistons to accomodate the valves have sharp edges, and there are sharp edges around the plug and sometimes around the valves in the heads. All these edges should have been grinded away to produce an absolutely smooth surface - this is to avoid hot-spots which cause pinging.

Then, the lightweight pistons that 7-rocks sell does creat sounds that stock does not, nor does Venolia forged pistons either make these sounds. The sounds could be a "personality" of these pistons - but that is for 7-rocks to lay out / describe.

As for the oil... that is bad.
Usually it means leaking guides... but with that amount oil oil being burnt you are running almost a 2-stroke... and that does indicate a ring problem. However, it can be a nicasil problem - if you by any chance have damages to the nicasil that can cause an excessive oil consumption.
There is a third very perculiar option to - valve-cover ventilation problem. Friend of mine had oil consumption like yours and in his engines case it simply had something to do with poor ventilation of the valve-covers (I know this sounds weird, but I have pic on a sidecar racer with valve-cover ventilation, so the problem is real).

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Drive Safely,
Albert