Sorry to hear that you are having issue, that is never fun!I would wait a year or 2. Let JVC iterate on this new platform once or twice. That's my plan. Maybe they will drop the pricing back down to previous gen levels as well. I'm not too hopeful on that one, but could happen.
BTW, I'm having more trouble with my DI on my 540. I'm thinking mine may actually be faulty now. It seems to get to a point of causing a brightness pulsation, not just pumping. Need to test more, but pulling power to the unit seems to temporarily fix it. IIRC you gave up on yours which is why I'm mentioning it.
Being new to JVC when I first purchased the X790 I really didn't know anything about the DI and how it worked. My first X790 did have a faulty DI that was stuck and not working so that was replaced. When I got my replacement I started noticing pumping on HDR material and worked with JVC and they also replaced that unit.
Now I am on my third X790 which I do not use the DI at all. I have my SDR set to -12 and my HDR set to -4 and just leave it that way and have been perfectly fine with it. I miss the fade to blacks but do not need to deal with the pumping.
Over time I have learned that the X790 can not use the DI in HDR without getting pumping. The steps between the DI are just to great and it is not fast enough to compensate for the quick dark to light scenes. For SDR material it will work fine but having the iris set to -4 or greater is just too much change for the DI to handle and that is why we see the pumping.
I am pretty sure this is the reason the new projectors have the DI set to be much less aggressive.