This hack only works in the “trivial” case that your multiple WANs are in the main routing table.
I have a setup where I establish my WAN uplinks via VPNs (also wireguard). Then you need policy routing (VRFs don’t work because not implemented) and everything goes nuts.
If you have that setup, check my solution: http://forum.mikrotik.com/t/routeros-blatantly-ignores-pref-src-can-this-really-be-a-bug/180360/1
PS: But I am close to abandoning wireguard altogether. I think it’s nice for a trivial client-server setup. But once you bring PBR/VRFs etc into play it’s just a mess and their “design decisions” not made with these in mind. For example, a service that insists to bind on every single address on the system is just plain bad.