Oil was leaking from underneath the oil pump and on to the manifold, so I changed the front oil crank seal. But now the leak is even worse... I didn't change the O-ring behind the oil pump, could this cause similar problems?

Or is my engine simply worn out, causing blowby? It only reaches 4.1 bar@100 degc oil temp, no matter what rpm. 127.000 miles, used as a track day toy since new.

 

O ring??? Get some Hyper gasket remove the oil pump and reseal it.

 

There is an "O" ring between the pump and block face but the oil pump back plate screws might have come loose and leaking from there - quite common to happen.
If it has been used as a track day tool from new then its a fair chance you will have excessive blow-by. Get a cylinder leak down test and a Compression test done to see what the Internal health of the engine is.

Mick