I'm going to side with the Railroad and did pretty much all the BoS quests I wanted to do. I'm at the Mass Fusion quest now, which will make the BoS my enemy (permanent, I take it?). Are there things left to do or find/take before I turn on them? Things or items that will not be accessible when I do? Thanks in advance for any help

Make sure you buy all of the unique guns from Teagan. Also the vertibird grades. And steal the stuff in the back of his shop. Take your power armor also if you have it docked there. Turn in technical documents and viable blood samples. Also, if you turn in enough viable blood samples, you get some stuff that's better than RadX, but it's not a big deal. If you haven't brought back Paladin Brandis, do the mission now. He gives you a unique laser gun afterwards if you convince him to rejoin the Brotherhood. Also, if you haven't gotten Paladin Danse's perk, it might be too late to get it now if you haven't done Liberty Reprimed. You need to do Liberty Reprimed first before getting Blind Betrayal, which is needed for Danse's perk. Since Liberty Reprimed and Mass Fusion both require talking to Proctor Ingram, and you tell Proctor Ingram immediately about the agitator, you can't do Liberty Reprimed first, even if you talk to Ingram right after warning her about the agitator. If you're not at the objective intersection though, you should be good. Also, Lancer Captain Kells has non-repeating side missions that are easy to miss since I personally rarely visited him. One of those missions may require you to cure or kill Virgil first before doing, though ?probably? not. I honestly don't know if there's a trigger besides meeting Virgil and having hiim help you in.

That's all I can think of on the top of my head. I'll edit later if I think of more.

Thank you for your detailed reply! Got pretty much all of that checked, also I have already done both Liberty Reprimed and Blind Betrayal. I know Danse isn't gonna like it, but at least he's safe due to my actions.

Virgil is safe and pretty much back to his own self. *censored* Captain Kells, he's going down.

Do you know where I'll find Piper after Mass Fusion and if/how she'll respond to my new stance with the BoS? I know she doesn't like them, but not sure how she'll feel about being enemies.

Not the same thing, but I just tested her reaction in my save that had the Institute ending. She expresses extreme disappointment that you helped the Institute, then completely forgets about it when you talk to her again. She'll still be willing to follow you around, so that should be consistent even if you betray the Brotherhood. Oh, and if Cait is your current companion and you switch to Piper, Cait informs you both that she's up for a threesome.

Lol, yeah I noticed that with Cait. Most of Piper's remarks when switching to a different companion and back are funny in my opinion.

Hope you don't use him as a companion as he will hate you for betraying the BOS. I think he will outright go hostile on you or at least refuse to follow you.