So as for your buys, particularly with Villages, one general piece of advice I can give you is to divide your situations into two categories:
First, where the village split matters. If there's only one stack of Villages and it's important to win the split, then usually you just build your deck early on where winning that split is a priority, so overbuying is not as bad here (but don't pass on early trashing or junking or anything crazy just to win the split).
Second, where the village split is less important. Let's say there just aren't that many terminals you need to play, or there are multiple villages on the board so denial isn't a huge deal. In this case, you'll often hear me ask myself "what is the best card for my deck right now?" Now what this really means is "what are the best cards to put in my deck before I shuffle again?" and you'll want to track your deck to see what you're likely to buy on your other turns before you shuffle, but this is much more a short-term decision. You should have an idea of what you want your deck to look like, yes, but you'll want to add cards to your deck that get you there as quickly as possible.
If you're drawing your deck, it's a simple decision -- increase your payload as much as you can while still being able to draw your deck. You can actually plan a few turns ahead in these cases and get some extra benefits. It's pretty clear in this case that you don't want extra Villages, because extra actions at the end of your turn don't help you at all. If you are on your way to drawing your deck but not quite there, this still mostly holds, but you can't be as precise about your payload and planning ahead usually isn't profitable.
If you aren't drawing your deck, but plan to some day, then maaaaaybe you want like one extra Village for reliability, but this depends on a lot of things and will usually not give you a super-huge benefit. Certainly if you're properly balanced now and you can only get one card before you shuffle, Village is probably better than payload, but again this is not always the case.
If you don't plan to draw your deck, well, you should think pretty hard about whether you want Villages at all. Maybe you still do, but a lot of my advice doesn't apply anymore in that case.
As for not triggering the shuffle, I mean I've gone through a progression of stopping myself before playing all types of cards and making sure it's a good thing. And yeah it's really hard. There are still tons of things I realize I did wrong right after I clicked it. It will probably take a while to get used to it, but the right answer is to think before you play your cards always.
Yeah this takes forever, so I've developed some habits. Like I just try not to click the "Play All Treasures" button when Farmland is on the board (and Save -- Iso bailed me out several times on this one but MF Dominion probably won't) and other assorted things. You can just try to put them into your mind when the game starts along with other strategic points (think about gaining Inn at all times, etc.) It's a slow process, but I think it's a good way to get better at the game.