It is now sometime in May and I can report on my feelings on the ending! My feelings are... the game doesn't make you feel great about winning, does it? But it's a good and really interesting ending nonetheless.
Tons of spoilers below in tag:
Having the Institute run by Shaun was a very smart stroke on the part of the devs, I think that was an extremely well played plot turn. That said, I think they needed to play up Shaun's attachment to you as family more to humanise the Institute, whereas it felt like that was a bit too expected from the player. Shaun's familial attachment is plot-crucial, because it's the reason he puts stupid amounts of trust in you despite that being obviously daft, but it doesn't come through in the acting as well as it could I think.
As things career towards conflict, I did get strong senses of "I am surrounded by idiots and I cannot explain this fact to them". Every faction is supposedly pretty good at intel, but every faction is nonetheless underestimating all of its opponents' capabilities to a ridiculous degree, and the endgame is nowhere near worth the risks taken. I think maybe the Institute's master-plan needed to be much more destructive to the surface or something? Not sure. But the Institute's need to control synths is not really very well explained, it just ends up seeming like a cult-obsessive feature which ultimately brings an immense amount of destruction. The Brotherhood make more sense in this regard since they actually are a mad cult whose hubris knows no bounds, but the Institute's arrogance and blinkeredness hangs together a little less well.
The minutemen/railroad side is also a bit odd and the relationship between the two feels unresolved (though shout-out to the Radio Freedom broadcaster who does a very earnest job trying to rehabilitate the railroad's image post-game). It's weird that the railroad occupies tons of open checkpoint locations post-game: it makes zero sense for them to do this rather than the minutemen, and it might have been nice to have a "deciding the future" quest/scene where you have to sit down with Des, Z1-14, Preston, and maybe the mad explosives minutemen lady, and thrash out a plan, assuming you're sufficiently pro both minutemen and railroad (part of the oddity here being how much the minutemen are a True Neutral faction). I did keep wishing that, as the Minutemen's literal leader, I could get my teams working together better (I used Minutemen to get into the Institute the first time, then switched to the Railroad plans for destroying the BoS and Institute).
I'd also have liked to have a bit more show-not-tell on the Railroad's achievements after: unlike the other factions just showing them at checkpoints seems weird, and it'd be conversely good to have a bit more visible contact with the supposedly many synths who are being transported out. I guess the achievement feels impersonal, whereas the costs feel extremely personal, especially Liam's suicide which definitely felt a majorly jarring gut-punch. Not an unearned gut-punch, mind you, but it felt to me like there was a mismatch in that the punches landed heavily but the rewards felt thin.
Anyway, definitely enjoyed Fo4, will be up for playing more Fallout games at other times though it's a fairly long game series so probably won't do that immediately. I'd like to pick up the DLCs for Fo4 too, my guy Hannibal still has some story left in him I think (though he's one of my more generic protagonists by and large, just a smart and somewhat charismatic military leader really).