I just finished

for the BoS, I'm into the institute but didn't side with them just yet, I've gotten to "taking independence" with the minutemen and built the artillery and I've done a few quests for the railroad like getting and I've done a TON of side and radiant quests so far.

I'm left wondering how much storyline I have left before I'm forced to pick a side and what happens when I do beat the game? can I still keep playing? will I have to reload a save in order to do the other storylines? is there a new game+ that lets me keep my goodies that I've spent way too many hours collecting?

Not long.

You can finish one more BoS quest before things reach a decision...

(save regularly)

ya I did a few things, eliminated the railroad and then decided to go with the institute, I mean, it's nice down there, fresh air, green plants, complete security, and teleportation technology.