I let the ECU enrich the fuel mixture with fuel, but the enrichment should never reach such low lambda values as this OEM is the case.
Wrong. It will also go 0.75 or lower lambda OEM.
The experience of my engine builder has shown that full load driving with too low lambda value can massively reduce the life of connecting rod bearings and the life of the oil (depending on how long the full load driving with very low lambda value is done and how much the oil is damaged).
Your "builder" has 0 understanding of combustion process. On WOT the mixture richness is not going to do anything.
The problem is on cold start when the fuel does not evaporate. Nothing happens to the oil running rich on WOT because the engine is hot and unburned mixture goes to exhaust. It does not get deposited on cylinder walls or in the oil.
Claiming this is ridiculous.
You invented an artificial "problem" and trying to solve this "problem".
Your solution is not inventing an artificial "problem".
Comprehension question about TABGSS
If the EGT temperature exceeds the threshold of 979.99 degrees Celsius in TABGSS and the engine load is greater than the value in KFRLATR, the fuel mixture is continuously enriched, in extreme cases up to lambda 0.6? Are the limitations for lambda target (interaction of KFLBTS, KFFDLBTS and KFDLBTS) not considered?
There is a minimum lambda target. Overall you should tune the engine so that it does not need 0.7 and lower lambda to keep EGT in check.
If this is not the case, then you are trying to solve an issue with tuning that was made by your super duper engine builder, who built the engine with too high compression ratio for the required boost.
This causes you to run too late timing and your EGT problems.
You are not going to solve mistakes in hardware with software mapping. Take the heads off, install compression dropping headgaskets, and your problem will magically be solved.
Instead of further theorycraft start posting logs. You have hardware issues or huge issues with mapping to cause high EGT. The RS4 does not have EGT problems.