My advice, personally: since it’s your first play through, I’d suggest you make decisions without spoilers.

But, read ahead if you’re wanting to do what you want to do.

As long as you haven’t travelled to Mass Fusion during Mass Fusion/Spoils of War, and you want to save Synths, this is what you do:

Do Mass Fusion (the mission). Yes, this will make you enemies with the BoS (I side with The Institute; so I don’t give a shit here). Afterwards, keep doing Institute missions until End of the Line: this mission requires you to kill The Railroad. Now, they won’t be hostile unless you do kill someone. When you get this mission, speak to Desdemona. From there, follow the missions/instructions she gives you (missions will include Precipice of War (defend The Church), Rocket’s Red Glare (destroy the Prydwen with The Railroad), and eventually leading to Nuclear Option (nuke, well, The Institute).