Wow, this is impressive!
Here's some feedback. Sorry if it's a bit harsh, and a bunch of it might be incorrect.
Attendant
Attendant looks a bit automatic. I think it'd be much more interesting if it wasn't a cantrip (+$2 instead of +1 Card +1 Action, for example).
Barkeeper
The choose one on Barkeeper doesn't make sense. If it's purpose is to do something on the Tavern mat. there's not much point, since +1 Buy is such a weak bonus. I would recommend either removing the +1 Buy, or making it happen regardless of putting it on the Tavern mat. Right now it adds needless complexity. I also don't understand the "return to your Action phase" calling. This basically does not help you at all, except for edge cases. All official cards that have this effect also either give you Actions to use (Villa), Cards (Cavalry), or both (Launch). Switching phases has no inherent good, it's only good if you get more stuff to use.
Here's a suggestion: "+2 Villagers, put this on your Tavern mat. / At the end of your Buy phase, you may pay $2 to call this. If you do, +2 Cards and return to your Action phase."
Brewer
This card has too many concepts squished into one. It's a cantrip Villager/Market Square/Reserve Groundskeeper. That's way too much needless complexity. I would recommend something like "+$2, this turn, when you gain a Duchy, +1 VP". The Villager doesn't make sense here.
Farmhand looks super weak. If you're using +1 Action you shouldn't have bought it (Ruined Village sucks); and Ways are really bad to have in your deck. The thing that makes Ways good is that you can use any normal card for them. Yes, this lets you choose from two but most of the time I don't want this for free. I'll look at some Ways:
-Butterfly, Rat, Turtle, Pig - pretty much useless
-Ox, Otter, Squirrel, Worm, Sheep, Monkey - really bad
-Goat, Camel, Owl - could be situationally good, but if there is any alternative this is terrible
Edit: and Farmhands is an official card
Gamekeeper
I would remove the optionalness of Exiling; you're buying this for the points, not for a cantrip or a Moat.
Robber Baron
This feels like a weak Throne Room. By the time you don't have Coppers to discard the Copper gaining won't help, and not doubling $5s is a huge downside. It could cost $4.
Seneschal
By now I've seen so many variations on choose one vanillas that they've gotten unexciting. One of the things that makes Liaisons cool is that they al provide favors differently. Playing with e.g. Bauble, Syncophant, Student, Underling, or Emissary makes Favors feel like a different resource each time. Your cards just give them automatically. Commune, this, and Majordomo all feel redundant.
Waresmith
I think you underestimate how terrible a +1 Action card is. If you're getting +1 Action with this, you shouldn't have bought it! That choice doesn't add to this card. So many of these cards feel like random vanilla bonuses tacked onto choose ones and conditions.
In general, my feedback for the Action cards is simply to make more unique cards, where there is a simple, concentrated idea behind them, instead of variations on the same idea where each card doesn't really have a core identity.
Treasures
Beer
Great card. Simple and effective. A few problems though: (1) don't draw cards in your Buy phase. It's no fun to draw Action cards dead with unspent Actions. You can remove the +1 Card and this is still fine. (2) Putting it on the Tavern mat doesn't need to be optional. You're almost always going to gain a card, so you can call it this turn if you want it back for some reason. If you think that's too weak, make it +2 Buys.
I suggest "+$1. Put this on your Tavern mat. / When you gain a card, you may call this for +1/2 Buy(s)."
Compost
Wow, this is cool. Only problem is that Exiling from the trash could be unbalanced in games where Provinces end up in the trash (Barbarian/Locusts).
Hearth can put the card in your hand at end of turn instead of the start of your next turn.
Keg
Cards in Buy phase = bad! Sorry, this is a pet peeve of mine.
Scarecrow
You won't be gaining Curses. This is a weak Stockpile.
Waterwheel and Viaduct look like Silver and Gold respectively.
Brooch, Keg, top of Salt, Scarecrow, Scythe, Wheat, Wine, are all very repetitive. They're more vanilla stuff with choose ones/conditions. They don't add much to the game.
Victory cards
Chateau is fine, but do you really need two identical overpays in the same expansion, one of which is a plain 2VP?
Granary looks weak. $7 for a Duchy, $10 for a Province, $15 for a Colony. Maybe the flexibility and trashability makes up for it; it looks interesting for sure.
Plantation's card does not match your comment. With one empty pile, the card says you'll get 4VP, but your comment says 7VP, and you'll always have at least one, so this looks super weak to me as what's written on the card. If I go by the comment, then it looks interesting.
I've made a card that's the same as Wasteland, but with an on-gain effect. I like it, so I like Wasteland.