IMO Asus makes some pretty excellent hardware. There strix line of graphics cards, while a bit spendy, have some of the best coolers available. Just because you bought the lowest end card they make, doesn't mean they make inferior products. If you knew msi and gigabyte make better cooled versions of the card, for less money, why did you buy the Asus one in the first place?
That being said, the only 1660ti block I found after a quick Google search was made by alphacool. Unfortunately your particular model was not on the compatibility list. But if the pcb is a reference design, it may still work.
If you want a custom loop for both CPU and GPU, you'll be looking to spend well over $500. Just the GPU loop, if you can find a block that fits, will be $350+ .Its definitely not budget friendly. Like mentioned above, your best bet may be to sell your current card and buy a new one, with a better cooler. You'll probably lose a few bucks I'm the process, but it'll be cheaper than a loop.
View: https://youtu.be/WNZjsAzhzCo
This video shows the very small difference in cooler design vs temps.
More than likely the best cooled 1660ti will be the strix card, or possibly evga xc ultra, both just over 300 on Amazon currently.