Does my iMac have a PCIe slot?

That may not be possible for several reasons. This firm is a popular source for Mac upgrades:

https://eshop.macsales.com/shop/ssd/owc/imac-21.5-inch/2013-2019

but has this caution:

NOTE: Aura Pro X2 SSDs can only be installed in 21.5-inch iMac models that came with a factory-installed SSD or Fusion drive.

So that won't work. Maybe another company has ones that works. However, I read that as Apple either did not install or did not activate the SSD socket on 21.5-inch iMacs. You can call OWC and ask them to clarify. They have great pre-sales support.

⚠️ However, and this goes for any internal work on 2012 and later iMacs, they were never intended to be opened by the end user. Breaking the seal on the case is a delicate operation that, done badly, can destroy the computer. There was a post here within the last two weeks where that happened.

You will need professional help; Apple won't touch it, and there are some Apple Authorized Service Providers who won't either. Expect US$150-200 in labor on top of the cost of hardware if you find an Apple Authorized Service Provider willing to attempt it.

What makes sense from both a cost and serviceability viewpoint is an external 6GB/sec SSD in a USB 3 or USB-C enclosure set up as the boot volume. That will give more capacity and, when you boot from it, your data transfers will be about 8X faster than you now see.

Not sure what ports you have but I run an NVME WD black drive in a TB3 enclosure with my iMac and it is faster than the internal SSD. Opening up the iMac is very tricky to do.