I'm not at the point, where I can choose a faction in the main story yet

but I have an option to join Paladin Danse after that one quest at the beginning

and I'd really like to get that Vertibird

will I have a chance later in the main quest, to abandon the Brotherhood and side with someone else?

or will I be bound to them, if I choose them early?

You can safely join all factions and do quests for them. There are many quests and actions that can turn you hostile to factions and allow you to side with others.

Heck, you can finish the MQ with BoS and then destroy them with the Minutemen. It is quite dynamic.

alright thanks