it is possible to route a subnet that is accessible only via another openVPN client using ccd scripts
For example in your case, you would have to add on your AWS server configuration resembling this
Let's assume that 10.80.0.1 is the IP of your AWS VPN Gateway (The VPN address, not external IP), and 10.0.2.0/24 is the subnet that you wish to route via MacOS server
iroute 10.0.2.0 255.255.255.0
push "route 10.0.2.0 255.255.255.0 10.80.0.1"
route 10.0.2.0 255.255.255.0 10.80.0.1"
You also need a line In your OpenVPN server main config file on the AWS server
What it does it tells OpenVPN server upon the
macos_vpn_commonname client connection, that the subnet 10.0.2.0 /24 is reachable via that client and enables routing via that tunnel. As far as I was testing there is no other way to do it, even manually routing the traffic via previously created tunnel will not work.
That would cover routing the subnet through a MacOS server.
If you need to redirect ALL traffic through it then let me know and I will try to help you further using iprule / iproute on the AWS OpenVPN server and
redirect-gateway directive, as I'm not sure if OpenVPN is prepared for such scenario with internal mechanisms.