What you'll need to do in your case is change the 0.0.0.0/0 VS from forwarding-type to fastL4 type, and attach a default pool, in which you specify the gateway of F5 (directly connected destination, via external selfIP interface).
- Make sure that 0.0.0.0/0 VS is listening on the Internal VLAN from which the hosts want to access the Internet.
- I also recommend a custom stateless fastL4 profile, instead of the default fastL4 profile for this kind of 0.0.0.0/0 VS. Have a look here for reference: https://devcentral.f5.com/questions/f5-big-ip-memory-issues