Unfortunately, NVMe SSD make RAM drives pointless. SSD, and any RAM drive, saturate SATA. NVMe make SSD perform well but won’t work for RAM drive since it is just a card. PCIe works, the old ones used it.
The PCIe RAM drive cards were enterprise and super expensive. You could buy a whole computer for one. It is probably cheaper to sell the RAM sticks and buy SSD. As I mentioned, they are super cheap now and cache would only need small one.
Also, setting up RAM drive is a pain. The old cards required special drivers; new ones could probably emulate NVMe. But setting up cache drive and getting programs to use it for temporary storage is annoying. All SSD is easier. There is a lot of experience with using SSD as durable write cache and not as much using disposable temp space.
For those who need speed, buying more RAM is a better way of doing it. If you really need speed, then new computer with more RAM and NVMe is better approach.
In any case, PCIe RAM drives aren’t made anymore. They were a niche at the time and now there is no market at all.