now I'm curious how that last turn would go. let's see.
after I play the scrying pool, my hand is 9 of each action, except 8 pools, 10 king's courts, and 10 throne rooms. without using the courts or rooms, or crossroads, or anything I might need later, I have the following non-terminal draw:
village (9 draws)
spy (9 draws)
market (9 draws)
wishing well (18 draws)
conspirator (9 draws)
mining village (9 draws)
Haven (9 draws)
Pearl Diver (9 draws)
Caravan (9 draws)
Bazaar (9 draws)
Treasury (9 draws)
Apothecary (9 draws, also clears out coppers)
Familiar (9 draws)
Worker's Village (9 draws)
City (18 draws)
Grand Market (9 draws)
Peddler (9 draws)
Hamlet (9 draws, probably discard for the buys)
Menagerie (unfortunately, only 9 draws)
Tournament (9 draws)
Hunting Party (18 draws if done early, so I can fetch all different victory cards)
Oasis (9 draws, discard something irrelevant, like coppers from apothecary)
Scheme (9 draws)
Cartographer (9 draws)
Highway (9 draws)
Border Village (9 draws)
Walled Village (9 draws)
Laboratory (18 draws)
Alchemist (18 draws)
Trusty Steed (2 draws)
Inn (18 draws)
Warehouse (27 draws)
Stables (27 draws)
that's 380 draws. add on the dark ages ones, I'm too lazy and I don't need them, but if I do they exist too. there's 13 victory cards, each of which I have 7 of, plus one knight. that means I blast through them to treasure pretty quickly, then I can use my crossroads to draw 92 cards each. that's another 828 draws. I think it's safe to say I have everything in my hand now. if not I can now play my Great Halls for 9 more but I doubt I need that. anything that can't be done with over 1000 cards can't be done.
anyway, with some basic TR/KC/Proc stacking, I can get 22 tripled actions and 17 doubled actions. I've played 9 Highways so I need to use one doubled action on bridge. now let's look at how many non-terminal buys I have: (counting the ones played in the non-terminal draw phase)
Festival: 9
Market: 9
Pawn: 9
Worker's Village: 9
City: 9
Grand Market: 9
Hamlet: 9
Forager: 9
Market Square: 9
that's 81 buys. plus, as NoMoreFun points out, I can use some of my KC actions on Squire, for 27 more. that's 108 buys and everything is free. then I can use the rest of my KC and TR things on terminal buy like Council Room, for 73 more. that's 181 buys. there's less than 181 unique cards in dominion, right? if so this is proof of concept that, at some point, this can be done. if not, upgrades haven't been played so you can get some more with that, and I have a lot of extra actions with which to play more terminal buys or gainers. plus there's some treasures that give +buys or gain things, so use those too. if anyone can figure out the fastest way to gain all but one of each card, feel free to staple this on the end to close the game out.