There is a long thread here about all things related to PCIe splitting: A Neverending Story: PCIe 3.0/4.0/5.0 Bifurcation, Adapters, Switches, HBAs, Cables, Backplanes, Risers & Extensions - The Good, the Bad & the Ugly
Apparently the cross-compatibility and quality are quite variable. PCIe <= gen3 seems easier than gen4, though, and for your needs it sounds like gen3 signal will suffice.
This thread and this one contain a discussion about, and in the latter case an example of, using the ASRock boards’ SlimSAS connectors to connect PCIe devices such as USB controllers. If these connectors are unoccupied in your system, that could perhaps be an option?