I think upgrading to a 6600 would be a good choice. That will net you a sizable jump in performance for the price. According to TechPowerUp, the 6600 is about 70% better than the 1060 6GB. If you were to upgrade anything else, I would say the power supply. Paired with the 8700 and 6600, yours will be perfectly fine. But if you were to upgrade to, say, a 3080 or above, then a beefier supply would be required.

Edit: The 6600 only runs on 8 PCIe lanes instead of the regular 16. This isn't an issue with newer hardware, but since your CPU doesn't support PCIe Gen 4 speeds (only Gen 3), you may experience lower performance. Idk if it will be significant. Mostly because 8 PCIe Gen 3 lanes should be enough to not bottleneck the 6600. I believe Hardware Unboxed, Gamer's Nexus, or another channel tested that, but I'm on mobile and don't feel like searching for it.

Besides a better gpu (look out for PCI-e gen difference) I highly recommend a higher refresh rate monitor, e.g. 144 Hz instead of 60 Hz. If you use your pc for gaming, this will make a huge difference in experience.

Also, unless you care about a silent pc or 'cool running' one, I'd wait and see whether you really need to upgrade your PSU when upgrading your GPU, but don't let it exhaust itself

The RX 6600 is really nice for the price. Nvidia wise that would be a 3060 (which goes for a lot more and is actually slower at times, though also has DLSS).

As you play at 1440p60hz it would be a really nice upgrade. Your CPU won't hold you back at 60hz :)

Depending on the type of games you play the 16 GB RAM might sooner or later become limiting (Absolutely zero issues with 32 GB), but it's going to be fine for most games. Especially if you're not a power user (like lots of browser tabs, Discord, several launchers, ..).

Having 2 1440's, yes, you badly need to upgrade video card. Something like a 6800 XT, 4070 may work or might not be strong enough but 4080 is a lot of money, or 3080 Ti.

After you buy a GPU, save up for new board and processor, your 6 cores won't always keep up with graphics card

You'll want to upgrade your "core" platform soon aswell but your current one should still be good for a few years. Really depends on what it is you're playing though.

Since you're looking at a budget video card, maybe have a look at some of Intel's Arc offerings. They keep getting better and better, and the prices are great.

Memory and storage are dirt cheap right now.

But also not really needed unless you have a need for it.

I upgraded to Rx 6600 a year ago, it's a nice card, unfortunately my XFX variant is very loud at 120W power limit

As someone who just upgraded from a setup with a GTX 1060 and a 1440p monitor to a 3060 TI - which is something like 25% faster than the RX 6600 - I would suggest getting something a bit faster if at all possible. The RX 6600 is still a pretty good card for 1080p, but the extra resolution is a bit too much with most newer games. Though if you are okay dropping to medium settings or utilizing upscaling it won't matter as much.