Diodes are nice. But an old-fashioned rheostat enables you to "fine-tune" the voltage to the least amount to provide the slowest speed at which the bump-and-go will successfully restart.
Agreed. But note the OP says he has variable voltage control using his DCS system...the problem is the minimum voltage setting of 5V. By putting in a fixed voltage drop of 0.7V, or 1.4V (2 pairs of diodes) or 2.1V (3 pairs of diodes), etc. he still has variable control to "fine-tune" the voltage. But now his minimum voltage starts at 4.3V, or 3.6V, or 2.9V respectively.
Separately. Confirmation needed from one of the MTH TIU experts but here's another idea. As it appears you're using a variable channel out of the TIU, note that the displayed voltage assumes a TIU input of 22V AC. If you supply a lower input voltage, then the output voltage is scaled down accordingly. So, for example, if you supply only 16V AC (or about 73% of 22V), then even though the DCS remote says 5V output, the actual output voltage will be about 3.6V (about 73% of 5V). So you can still "fine-tune" the voltage but starting from a lower minimum.