Relay Switching On and Off Rapidly During High Current Draw
First point. A solar cell is like a pn diode basically a current source up to a given voltage, above which the current reduces. When solar irradiance reduces, the I-V curve lowers. Please, have a look at a past discussion: How to estimate solar panel output as a function of solar radiation
I show the relevant image below.

Second point. The minimum voltage relay is proposed to avoid excessive discharging of batteries, to avoid to reduce the expected life. For the PV panels we do not have these problems. When sunlight is off they will not provide any current. I am not aware of any wearing due to low irradiance. Commonly used panels on roofs and PV parks are not switched off, but converters downstream are not operated for a  matter of efficiency and economy.
Conclusion. You can remove the &quot;relay&quot; from your scheme and see how things are going. The pump will not suffer of working at lower voltage and overheat because the converter guarantees the feeding voltage.
Remark. The relay would have been a good idea if there is some kind of hysteresis.
It was conceived for batteries, thinking of a constant discharge towards lower values. In your case you might have an increase of irradiance at dusk for some time, and also cause by moving clouds.
By the way even the buck converter will have a min input voltage, and will go off below that.