So I am level 37 now, just doing side quests and Minuteman shores. I have yet to start on Diamond City, Railroad or BoS. What would you advise this noob stealth sniper to do first?

a) Get to Diamond City. It's about time. Follow the freedom trail.

Possibly c) Once you get closer to the city center, north of it, you should get in contact with an emergency radio broadcast, leading you to some people fighting of a horde of ghouls.

Lol you sound like me I get so distracted. Hard to advise what next. I started chipping slowly away at Diamond City though. This game is so huge cracks me up hearing these "Jamokes" blowing through it in a few days. Try reading the consoles exploring it's crazy what you'll find.

hehehe damned Settlements take me so much time. I have a bit of OCD so I am never done with them. I am a bit confused though: is not the Freedom Trail a different quest than the Main Story (Diamnod City)? Or do they converge?

You're required to do it in the story, but it's not related to begin with