I’ve owned XC 90s since around 2014 and never had this problem

Sunroofs are not expected to be waterproof forever. That's why there is an inner channel in the roof that drains any water to one or more drain hoses that route to under the car. I'm guessing that yours are clogged, which I've seen before.

The video below goes through the whole process of accessing the hoses in the roof pillars - you probably don't have to do that! (curtain air bags!!)
I located the openings of the hoses around the sunroof and poked around with a wooden skewer, which did the trick.

Volvo XC90 Front and rear sunroof drain tube location 2003-2014
https://www.youtube.com/watch?v=X_eXgdwMzQA

You can buy a $10 sealant and apply it to the roof if you are ok with it being permanently sealed

You bought a Volvo. What did you think would happen?

Did you see if there is a clog? Try seeing if there is a clog and clean it out. There are a lot od videos on YouTube dealing with this issue. How much is your time worth? You can try a DiY fix if they are trying to rip you off for $6k.

Anyone have this happen in their car? Apparently, it's faulty design in the 2017 model; Volvo knows and acknowledges, but won't recall. They've quoted me $6k to fix it. It seems really insane, and I'd like to fight it somehow....the car is worth barely more than that now, but has no other issues and only 100k miles. I'd love to have it for my kids to drive, but I'm not paying to fix this issue (which again, Volvo admits is their fault, and not due to any problem I caused). What can I do?