Running pptp over wireguard is great, because while wireguard has no idea whether a link is up or down, pptp does, because it uses ppp, which has LCP, which has keepalives, yay!

And this way the openwrt on my router knows when the interface is up and when it's down and should be able to add and remove routes as needed, right?

Well, sorta, on the client end.

The server end is not integrated with openwrt's netifd in any way, shape, or form.