If they had a HAM in direct neighborhood, I imagine said HAM would already pay them a visit - the interference from the inverter is likely not constrained to the ISM band.

Not to be a pedant, but just for your info, Ham is not an acronym :)

I always saw it written as either "HAM" or "ham", and I assumed the latter is the "young generation doesn't give a damn about spelling or punctuation" spelling, and therefore that the former is the correct one.

On the cables coming in from the panels or wiring going back to the main panel?

I'd start with the wiring going back to the main panel first but be open to anything. Fixing RF noise is more of an art than science in my experience.