I don't know where this notion comes from that Photoshop is single-threaded. All you need to do is open task manager and you'll see the load spread over available cores. It's true that some tasks need to be performed in sequence, but that's due to the nature of the task itself, not Photoshop. That's the old joke about two women giving birth to a baby in 4.5 months.
But anyway that largely misses the point. Photoshop is generally not CPU-limited, but bandwidth-limited. What you're waiting for is moving large amounts of data through the memory subsystem. The crucial component is the scratch disk. That's where most of the action is.
What you need is a fast drive with lots of space set up as scratch disk. Traditionally that has been the bottleneck. There is no such thing as "enough" RAM, no matter how much you have! The scratch disk will always be used. Think of RAM as a fast-access cache to the scratch disk's main memory.
With the new ultra-fast NVMe drives (aka PCIe M.2), the need for huge amounts of RAM has diminished. RAM is probably still faster, but the difference has dropped to practically insignificant. It's a whole other story than spinning hard drives.
The main requirement of the GPU in Photoshop is reliability. Video driver bugs is a huge problem. They happen because drivers are mainly released to satisfy the gamers and let them run the latest games. The gaming community is generally not too preoccupied with correct protocol - so this tends to introduce bugs elsewhere. So they put band-aid on it and fix the bugs much later.
GeForces and standard Radeons are gaming cards. It might pay off to get a Quadro or a Radeon Pro. These cards are optimized for graphics, 3D and CAD, not games.