I don't think the oxygen sensor has any influence on starting. The control system ignores the oxygen system until it gets to operating temperature.

Check fuel pressure during hot restart. Most likely something isn't providing enough fuel under that condition. Maybe a tired accumulator or check valve?

An alternative to cranking for long periods is to crank in a short burst, count to 3, crank in a short burst, count to 3... I expect the engine will catch by the second or third try. Common M103 malady. There is an answer but I never found it. Part of the reason I got tired of my 91 300SE.

Sixto
... stay tuned