This is one of my favorite use cases and I did this as a customer.
For outbound traffic, you set up a vip on 0.0.0.0/0 and the pool members are the ip addresses of the isp routers. You want to set your vip to not perform destination NAT (no address or port translation) and your health monitors for those isp routers can be transparent monitors.
then you have your network point the default gw to the LTM/GTM.
I can explain more if needed but this should be documented so I recommend reaching out to your account team or finding those docs. Let me know by messaging me if you need more help.
Yes, you want LTM and GTM modules included on your device. You need the GTM part for inbound because you will have multiple IP addresses for each application you choose to expose (1 IP address per ISP, probably), so you'll need to have your DNS queries resolve to the correct IP address, depending on how you want to load-balance incoming traffic.