On a mac can I extend (not duplicate) my display to multiple external monitors?
No, second screens are addressed as separate windows and not spanned. You can run 6 plus displays on the macOS window manager even without external GPU, so it’s not about ports or cables or protocols. It’s the window manager design you’re fighting.
If you consider adding more pixels on a second or third display, then it’s trivial to add displays, just connect them. If you want a blended space where two displays extend into one logical contiguous space, that needs hardware or a different OS.  This came about in Mavericks and there’s no sign Apple is reversing decision on this.

https://www.imore.com/how-span-window-between-two-displays-mavericks

The best you can do is in mission control, enable displays have their own spaces.

how to make each display a separate space, yet have a keyboard shortcut to move both screens at the same time?

You also might be very happy with an external GPU since it simplifies cable management and unlocks performance far past the MacBook Pro / Intel chipset alone can deliver.