Have DHCP for Wi-Fi and static IP for Ethernet?
This Apple discussion sounds like what you're looking for, titled: Wi-Fi and Wired network load preference. This discussion shows a screenshot where you can augment the service order for the network connections.
Set your connections appropriately under their respective configurations (Wi-Fi vs. Ethernet) and then set the order so that the connection you've decided and configured as the Internet facing one is the one with higher service order than the other.
This should give you what you want.