This one falls into the "Heartbreak" category. Sorry to say, it appears the engine coolant was allowed to fall below its normal level leaving the engine susceptible to overheating, and it continued to the point where the condition warped the cylinder head.
Many of us on this forum are auto enthusiasts. We check things all the time; tire pressure, tread depth, oil level, brake fluid level and color, coolant level, etc, etc. Along the way we might spot a leak somewhere and attend to it right away.
But there are others, many others, who don't have the time nor the interest in such matters. They may have learned that the oil needs to be changed every 10k miles (or whenever indicated on the dash), but the coolant doesn't need to be changed out for 100k miles, so no worries. After all, this is 2021 and cars are made so much better than they were 20 years ago.
Some dealerships used to offer "Care and Feeding of Your Vehicle" classes to new owners who fell into that second group. I attended one such session even though I thought I knew it all (I didn't; actually learned something!). It was a good way to learn how easy it is to take the few minutes a month it takes to check stuff before things go south.
To OP (spmatus), you could check the National Highway Traffic Safety Administration
website for recalls and complaints related to your issue. I took a cursory glance, but was unable to find any similar complaints registered by other owners to the site (there were 246 engine complaints to comb through).
It is possible that the head gasket failed on its own (it happens) and that's why the coolant leaked to begin with, but at this point it may be impossible to prove. Obviously it would have been better if the low coolant level in the overflow tank was caught early before it caused engine damage.
My best advice would be to try negotiating with the dealer for a "goodwill" price on the repair, but to do so you will somehow (I know it's hard) need to lower your anger and frustration levels. If you love everything else about your Pacifica, let them know. I guess I'm just suggesting that you give them a chance to make the repair with the belief that you will end up as another satisfied customer. Or you could try finding an independent garage that can do a quality job at less cost.
And then, if you aren't already doing so, check the owner's manual to learn how to check your Pacifica and other vehicles each month to spot problems beforehand. Might save a lot of frustration down the road.
Good luck and all the best.