Please help me troubleshoot- Coast Spa heater or relay replacement

Hi all,

I have a Coast Spa Radiance Curve lounger I bought used 3 years ago. It has run without issue until recently when I found the breaker tripped and cold water. In troubleshoot the issue, I found both pumps run fine along with all jets, however once the heater kicks in, the heater relay sparks, and a little plume of smoke rises from it, and then the main breaker trips. I removed the heater assembly and there’s nothing visually wrong (no cracks/burns/wire issues). This spa uses the Balboa Revolution heater assembly and you can’t simply replace the element. Where would you begin with this repair? A new heater assembly is $400, a new control board is $800, a new relay is $10 but must be soldered into the board I have zero experience. I’m open to any and all