I don't think it's the fan, but that piddly little heatsink that probably can't handle more than 100w of heat, being made to handle BOTH the gpu core and the GDDR6 Vram.
That looks like the same aluminum heatsink + copper core that gets used for the Intel downdraft coolers.
No, 89C isn't safe. Put the temp limit back to the default 83C, please. If you could keep thermals under that, it would be a bit more reassuring.
I couldn't find the Vbios for the 1660 model, but the following is the 1660 Super one, which shouldn't be all that different:
https://www.techpowerup.com/vgabios/214985/asus-gtx1660super-6144-190911
What can you do? There aren't many options available, since the 'bottleneck' here is the heatsink.
If the heatsink's capacity is low, replacing the stock paste + the thermal pads for the Vram should enhance thermal transfer, and that's about it; they won't improve the heatsink's capacity.
Paste: It's gotta be something thick and sticky, like Cooler Master's Mastergel Maker, for example. The silicon die is completely different from the grainy cpu IHS - it's comparable to glass, and more fluid pastes like the popular Arctic MX-4, don't do as well here.
Pads: This one could be more trouble than it's worth, because the size of pads used is different between cards. You won't be able to find out unless someone has already measured them on a forum, or Asus is nice enough to tell you.
You also need thermal tape for use with the pads.
The single fan: Crank it up if you weren't already.
TL;DR: The fan is capable. The heatsink is another story. Restore thermal limit back to default 83C.