Don't be scared. I've been running my 4090FE since lauch and I have zero sag and the thing is solid as hell. It's built better than almost all other 4090 offerings.
Holy Calzone! Were you in a coma for 25 years? Did you defect to consoles then come back to PC space recently? Why such a large gap?
You might be the first person to justify a 4090 purchase as "Making up for lost time" 😯
First, do not "reflow" this card, assuming that by "reflow" you mean put it in a domestic oven. That's a good way to turn a working card into a dead one. The worst case scenario here is that you graft some case fans onto this thing.
What I...
Prior to a reflow, try pressing the card down by the corner while the system is on. Not super hard, of couse, but enough to see some flex. It could reconnect a broken solder joint. If the fans kick on you know that's what it is. Long shot, but...
You could try installing MSI Afterburner and setting a fan percentage just to see if they turn on. Just set it to some percentage and see if anything happens. If nothing, double check that a wire didn't get pinched or something.
This is a fantastic keyboard. I have one full length and 2 TKL and it's a joy to use. One of the TKL's I use with a Corsair lap pad for my HTPC. It fits snug but works great!