A6 2.5 AEL: No More Proper Pull-Through:-((

Hello everyone,

I've recently been having some problems with my car: an Audi A6 2.5 TDI AEL (200,000 km).

Error description:
In my opinion, the engine has less power throughout the entire RPM range. Also, it recently started leaking coolant from the filler cap. I had it checked to see if there was any exhaust gas in the coolant => the result was negative. The compression is consistent at 28-28.5 bar. I then replaced the cap and the coolant regulator, and since then, there have been no more issues (I thought the head gasket might be damaged).
How would you approach troubleshooting a loss of power? (Oh, and there are no error codes stored.)