Chain slips off biggest cog under torque
Your first image suggests the chain is not meshing properly with the chain ring.  After 8000 km you should be either just finished your fourth chain, or about halfway through the second (depending on if your manufacture says 2000 or 5000 km per chain.)   How long since the chain was changed?

Do a chain length check too - 24 half-links (12 pairs of inside and outside) should be 12 inches long.  Any more than 12 1/8th inches is too stretched.   Running a worn chain normally wears out the rear cassette, whereas the front ones tend to survive because of the greater number of teeth.

Finally - use a straight-edge and confirm the front chainring is not bent.  This can be seen by eye if you turn the pedals while looking down from above.  A deflection could cause a bad meshing.

Over-lubing itself is not as issue.  However it attracts dust dirt and sand, which can do mischief on the chain, accelerating wear.  Best practice is to clean the chain, dry the chain, and then oil the chain.