This kind of error already popped up in a different context: Plasma 2350 W wifi problems - #18 by SirFico
It is a lengthy thread and it was solved by an up to date firmware. Which you already tried.
If you read through the thread you will see that one of the Pimoroni guys did not rule out a hardware problem either. So it might be best to drop a note to support, linking to this thread and ask them what to do.
The trick with the lower system-clock is not as simple as it sounds, since changing the system clock speed also needs a change in the PIO divisor (the CYW43 has a PIO connection). But I would say that @DivingIvan is maybe pointing in the right direction. Pimoroni support should check the clock speeds in their Presto firmware. Maybe they are just too high.