I would say if you plot the results of the substeps for 0.8 mm elements and you get a flat line, you can assume that the time increment is not important for the accuracy of your solution.
Then you can go back to element size. How are you varying the element size? Look at the part of the model that has the highest accumulated plastic strain. Are the elements at that location being carefully controlled for element size? For example, if you are just doing a global element size change, but geometry in the region limits the ability of the elements at the location of maximum strain from changing size, then you will not get a clean relationship of strain to global element size. Try to use a local mesh control at the location of maximum strain.
Also, how does the accumulated strain plot over time? Is there a lot on the first cycle and less and less on each subsequent cycle? You could do the mesh study on fewer cycles to save time. For example have the end time be 5045 s.