If you want to use eGPU but don't have a thunderbolt port on your laptop, can you use a suitable adapter?

I plan to equip my laptop with eGPU. However, I don't have a Thunderbolt port on my laptop. Can I simply connect a USB adapter with a thunderbolt connection to my laptop instead and then simply use the eGPU?

No do not go. Not every Thunderbolt connection works, it also has to support eGPUs.

An Expresscard slot would still work (but no notebook has that anymore).

Theoretically via a PCIe connection, for example, the network card - but that is no longer really usable.

No, it won't work

Have you ever figured out what bandwidth would be left for the eGPU? The word for it is called "depressing".