Load Balancing Outbound traffic between ISP Links with Big-IP LTM. HOW ?

you could you a performance layer 4 virtual server if you want to use a pool. it will also enable local traffic policies. with IP forwarding you indeed need routes and just iRules.

the SNAT behaviour depends on the "Source Address Translation" setting. if this happens for traffic going in you might want to only enable it on the outgoing VLAN interface.

to enable it for some, but not on others you will need an iRule or perhaps LTP, something to try out.