My dog obsessively marks outside when we go for walks. Any suggestions?

This isn't entirely uncommon, especially in intact males. I would make a very clear difference between his "free time" to sniff and mark and do his own thing and how you expect him to behave on leash with you. I would make walking with you an obedience exercise. I allow my dogs some free time before we walk or hike and they are told to "hurry up" (which means pee and poop and do your sniffing). Once I say "let's go," it means to walk along with me and no marking, sniffing, or pulling. They don't have to heel but they must be polite. I see you got the Loose Leash Walking Online Course, it's exactly what I would have recommended for you.