How do I route all internet traffic from one Mac to another?

Solution 1:

The easy part is sharing internet without VPN. Let’s cover that first. Sign out of VPN and set up network sharing.

• https://support.apple.com/guide/mac-help/share-internet-connection-mac-network-users-mchlp1540/mac

Once that’s working, you may need to ensure your VPN allows split tunneling and get into very specific configuration details to pull this off but this is a very typical software defined network configuration. Mac 1 is functioning as a router and that router has a point to point VPN tunnel to another router serving the destination network segments you defined.