I just wanted to follow up and say that although I was never able to get my strongswan config working, I was able to get this up and running using the brew package ‘macosvpn’, which allows you to configure VPN connections with the native Mac VPN client through the CLI, and the OSX command scutil. So that might be worth a shot for anyone who finds this and is also struggling with Mac VPN issues.
The set-up was easy for me, this is basically the whole script for Mac:
brew install macosvpn
sudo macosvpn create --cisco [vpn config]
scutil --nc start [vpn name]