Camn, yes the Headlight Relay gets its current from the normally open position of the (middle) Starter Relay.
And, as you say, not just the tach, but headlight, horn, stop light, warning lights, and charging reference.
I wonder if the tach is the most sensitive to low or fluctuating voltage and may show a problem before the lights go out (?)
Gusse, the Bosch I have (from PyroDan Pruneske) are 0 332 207 307. I am not sure the difference between them but good if they are all 5 pin so they can be switched around if needed.