The PCB core design now fits many use cases. It's time to think how to source cheap enclosures. On the usual Chinese hobbyist sourcing sites the G311/G212 box with dimensions 115x90x55mm³ is widely available. It's a nice fit for a 100x80mm² PCBs (minus a few cm²).

Such a boxes are currently on offer less than $3.50 with "free shipping". A preliminary check of the "mechanical engineering constraints" looks promising (e.g. M16 cable glands for mains voltage and pump wiring fit in).

This box is maybe also a good fit for other control and monitoring applications.