Even before the more polished products existed, people have hacked together external GPU enclosures, often interfaced to the host's PCIe bus via ExpressCard slots or ribbon cables hanging off of Mini PCIe slots. And more recently, with the interest in cramming many dual-slot GPUs onto a single motherboard for cryptomining rigs, you can find cheap, "barebones stand-alone PCIe slots" (for lack of a better term) for hanging GPUs off of. A quick search turned up a couple of examples:
The problem with all of those has always been that they only worked with an external display - there never was any way to send the image back to the laptop's own display.
It actually is possible with nVidia GPU-s, but you'll be taking a huge performance hit as some of the tiny bandwidth is now used to send the rendered image back to the laptop and then to the screen.
No matter what, sending it back to the internal display is going to mean a significant performance hit. There's only so much bandwidth in a TB3 connection.
https://www.aliexpress.com/item/Mini-PCI-E-to-PCI-E-1X-Expan...
https://www.aliexpress.com/item/60CM-PCI-Express-1X-To-16X-P... (Ingeniously, on this one, they've re-purposed USB 3.0 cables to carry the high-speed PCIe lanes).