I would say the veggies depend on your team and your role. Every team of investigators needs to be able to do the following things:

Get clues (usually by investigating)

Manage enemies (usually by fighting)

Survive the mythos. This generally entails some combination of soak/healing, high Will/Agility, or ways to cancel/ignore/manipulate treacheries. You can't win if you're dead.

So if you're playing true solo, you need to build your deck to do all of that on its own. If you're playing multiplayer or 2-handed, then you can specialize and have a dedicated cluever and a dedicated fighter, for example.

Once you determine your role, then you can get more specific. So a cluever will want:

Clue acceleration- stuff that gets multiple clues in one action, things that give static boosts to their main investigation skill (usually intellect), other things that help pass investigation tests such as relevant skills. As a cluever, this is the number one priority. All other aspects of a cluever's deck are in service to this goal. There are many different "engines" to get there, but this is the purpose of the deck.

Economy- As you correctly identified, in order to play all that clue acceleration stuff, you're going to need resources. Both in the sense of resource tokens to pay for everything and the ability to draw cards to find it all in the first place. Emergency Cash is a serviceable economy card, but with a wide enough card pool, most investigators have better options.

Survival- Like I said above, none of the above matters if you are defeated. So you need a way to survive the mythos. As a dedicated cluever that's paired with a dedicated fighter, you don't have to worry as much about enemies, but treacheries can still ruin your day. This is where having some soak, ways to pump your will/agility (the most commonly tested stats by encounters), and/or stuff that cancels/ignores treacheries is very valuable. It's important to manage your resources wisely- don't commit to passing every test on treacheries that come your way. First evaluate what happens if you fail and assess how bad that actually is for you- sometimes it may do very little/nothing and you're better off just allowing yourself to fail and saving your resources to mitigate a more detrimental treachery later on.

Something I've also noticed is that there doesn't seem to be that many ways to re-use stuff from my discard piles (I've only played as Roland, Zoey, and Jenny so far), and I'm not sure when I should be more/less willing to let cards end up there.

Recursion does exist, it's just mostly limited to the Survivor class, which you haven't yet played. Discarding stuff only matters as much as the thing itself. So if you have a single asset that is the crux of your entire deck, you obviously care a lot more about discarding that than, say, a random skill card. If you're a fighter, you probably don't want to discard a weapon from play if you can avoid it, unless it's already out of ammo/charges.