Regarding endings - on my first char I sided with the Brotherhood but blew up the Institute with the Minutemen, as the Brotherhood were starting to rather annoy my char, the business with Danse and the Railroad was a step too far..

What do I have available in terms of faction content? I mean, do I still have, for example, radiant quests with the Brotherhood? Or story content? I'd be a little pissed if siding with the Minutemen right at the end meant that the only repeatable content came from Preston...

If you side with the MM instead of the BoS, you still have access to the quests (assuming you don't blow up the Prydwen):

Quartermastery (available at Cambridge PD)

Cleansing the Commonwealth (available at Cambridge PD)

Squire Escorts

Scribe / Research Patrol Escorts

Blood for Neriah

Coolant for Ingram

Documents for Quinlan

... and access to Teagan's stocks, but not the gear that he reserves for Sentinel ranked characters.

So the only downsides are (understandably) not being able to advance beyond Paladin, and everyone aboard being a bit miffed?

Basically. Those are mostly grunts, though.

Quinlan actually quite likes it (as there are fewer of his brothers and sisters dead at the end).

Ingram's cool with it; 'If you get the job done, it's ok in my book'

Teagan reminisces with you.

Cade's happy about it (of course the medic's glad there are fewer casualties).

Kells grumps a bit, but goes back to giving out the Squire quests.

Maxson has no scripted lines, sadly.

Also, afterwards you have the Minutemen holding checkpoints and the BoS doing air patrols. Between the two, the raider, supermutant, and ghoul population near populated areas has nosedived in my current playthrough.

That last part sounds amazing.