How do I assign certain routes to each of my interface ?

Hello!

You can use SD-WAN Rules to route the traffic as you which, such as doing fail-over or even use SLA - to manage traffic for both WAN's. (SD-WAN Rules based on SLA is currently only available in v19, but since you're a home user I recommend you to update to v19 EAP 2.)

You can find more information at the Docs, there's enough explanation on how It works and the basics of It. (And some examples too) => https://docs.sophos.com/nsg/sophos-firewall/19.0/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Routing/SDWANPolicyRouting/index.html

Thanks!

Thank you for your reply. I have some questions to ask around this:

1. Do I have to create 2 SD-WAN rules: One for inbound traffic and another for outbound for each objective ?
2. Does it involve messing with scary-sounded things like OSPF, RIP, BGP, etc. ?
3. Can I also apply certain SD-WAN policies to the VPN functionality ?

Thank you very much again buddy. 

1) Only for outbound traffic.

2) No.

3) Yes, for both IPsec and SSLVPN.

Thanks for your quick reply buddy.

One more question, can I use the Policy Tester to test SD-WAN policies ? If not, how to test them ?

Hi Prism buddy, this is my SD-WAN policy. Does this look good enough ?