A PCIe switch does just that. It connects at its native speed then each port is backwards compatible effectively making wider effective slots if you connect older things downstream.

It can be hard to sort between expensive/high end enterprise stuff from low end single lane stuff for crypto mining when trying to find something useful though. NVMe m.2 storage is the best chance of finding cheep consumer options but even then you have to sort through the endless bifurcation cards to find the ones that are actual switches though.