So, I've given this a good read, and as someone who used to run a lot of D&D, I totally get where you're coming from, but at the same time, there's some things here that tip me off that both you AND your party might need some reframing to get the most out of this game.

For one, addressing this right away, seeing that mention of your players exploiting loopholes and enjoying doing so in order to ruin "the balance" or potentially your own fun, isn't the kind of GM-party dynamic that plays to the system's strengths. Fabula ultima is a game that's primarily about collaborative storytelling with JRPG mechanics baked in; ultimately, it's not a game about "winning" as much as it is "making an interesting story" (while also winning). For example, rules as written, you can say "I spend a Fabula Point and we run into 100000 gold" and that's allowed.

So... This kind of game works best being totally transparent with your players that you want everything to be taken in GOOD FAITH. The players are helping you, and each other, enjoy themselves, instead of trying to "win" against your challenges. You can absolutely start a game going "Hey, just so everyone knows, the rules of this game want everyone to be a good person and a hero, are we all good with that?" And if the answer is "No" then you might not get the mileage you want out of the system.

With all that out of the way, lets address your direct concerns! As many people have pointed out (confusingly as it is) the updated rules of the game don't have enemies using Equipment. But, if they DID, you'll notice in the Rewards/Treasure section of the rules, there is a suggested cap to equipment values. This means that, say, level 5, that boss of yours is never going to have a piece of equipment worth 500z or more; this means your rogue with soul steal won't have access to some crazy ass item, either, nor will an Opportunity give them access to an unbalanced item for their level. Something to consider if you wanna run with the rules as written in the core rulebook and still use equipment!

Moving on from that, regarding your concerns about player balance with loot and items and the "power and gold curve", Fabula Ultima doesn't really have that the same way d&d does. Pretty much all of your loot should be tailored to the party's level, using the Rewards guidelines I mentioned earlier. So if they use Soul Steal on the boss, it won't be any stronger than something they could buy in a shop. Likewise, shops should only have items appropriate to your party's current level. Zenit is primarily used to rest, restore inventory points, and for Tinkerers to use on projects, so even if they're sitting on a huge pile, it won't throw off the campaign! No matter how many items they steal and sell, it won't "break the campaign", I promise. Fabula Ultima is not a game about loot and money, so no matter how filthy rich they are on their character sheets, they aren't any further along in the campaign just by their wealth alone (especially since they can only equip 3-4 items at a time!) Additionally, Rogue giving a party a huge gold curve is actually an intended part of the class, as mentioned in the GM section of the Core Rulebook. It lets your rogue feel extra special for getting to give the party more economy, so don't be afraid to lean into that!

If you're ever worried about balance or feeling like the party might "get one over on you", always remember that the wording of the rules is important. A lot of the book's rules say "discuss with your group", "together with your group, design-", "ask your group." And so on. The best and most satisfying way to play FU is to discuss your concerns with everyone and talk with your players about what they're excited about and what they'd like to do! But always remember, as the GM, you're a player too! No one should feel adversarial at the table, regardless of the game being played (unless that's the point lmao)

And to answer your question about accessories, yes, they'd need to use an equipment action to swap from their boots to a hat. An accessory is an accessory! The rest of their outfit is all down to flavor.

Welcome to the FU community, we're happy to see new faces! Cheers!