Aged P i have tried that trick but still no good.
I think it maybe a build of something over time but if you see in the pics i still think the levels not right.
In the pan the water is a few inch's high even after not using the toilet overnight and if you take a straight line from that level to the soil pipe going out of the wall thats is where i think the problem is as water finds it own level.

The pipe is blocked, which is why the pan fills to the top.

I asked for a pic showing the side of the pan, the reason being an siphonic pan would do that if the bomb and half moon washer was worn or not positioned correctly. You have a normal pan.

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