Can’t get over 3000rpm on my 912a2 engine

Hmmmm! -  sounds like something not working properly on one cylinder (it feels like it’s running on 3 cylinders )

Fuel mixture etc would not produce the symptoms you describe.

Faulty plug, plug cap, lead, ignition module, would, as would a valve problem.

Suggest - Cylinder leak down tests. Check plug leads are actually "seated"/connected in plug cap.Plug leads have continuity. "Good" spark on every plug (I have had a brand new dud plug)

Has it just suddenly started doing this or was it a gradual change?

Have you tried pulling the plug leasds off each cyclinder in turn to see if you really have a non-working cylinder? If it is a single cylinder then you should do a compression test and a leakdown test. I have also seen a worn/deformed inlet valve that sealed sufficiently for a leakdown test but didn't seal well enough when the engine was running and causes a serious vibration. You may be able to inspect the valves and seats through the manifold connection apertures in the head, otherwise you may need need a boroscope.

What's the fuel pressure like? If you stop the engine suddenly at 3000rpm and then look in the float bowls, what is the fuel level like?

A simple check of the cylinder heads temperature will tell you if that’s the case. Carb balance is something to consider too.