Another option than the mentioned CM4 module might be some RK3399 based SBC, such as the Rockpro64, which comes with a PCIe x4 interface (the Rockpro64 has a open PCIe slot so you could put in a x16 card too).
If you just want to hook up some M.2 NVMe SSD there are also other SBCs with the same chip such as the nanopc-t4 that come with a M.2 slot.
If not I'll use Intel Atoms with SATA as backup to my Raspberry 2/4 cluster.