The basic config is two mangle rules and two items in routing table, that’s really simple way for you. Sure, I can imagine some even simpler high-level interface made specifically for people who need two LANs with two WANs, where you’d just enter addresses and it would set everything under the hood. But that’s not what RouterOS does.
Thanks for answers, looks like then I do need to add marking rules to route. Was looking for a simple way of just writing a rule to send via a specific gateway based on source IP.