The joy of many open-world games is the unique stories we tell from the different ways we can approach these games. But if I may be so bold, I bet your experience with Tears Of The Kingdom's weapon fusing was the same as mine. You fused a sword to your sword, thought it was funny (and also what the game wanted you to do), but eventually wondered what would happen if you tried one of those monster parts you'd been collecting. Your new weapon had great stats and a unique name like 'Reaper.' It was wonderful.
But then, suddenly, the honeymoon was over. You didn't have to experiment anymore, had no reason to ponder what else is out there, it was comfortable and reassuring to just pick whatever monster part had the highest fuse power every single time. No more rock hammers unless the game literally threw them at you, no more mushrooms on branches, no more steaks at the tip of your spear. And sure, you can fuse an arrow with a cooking pot to shoot it at Ganon, but did you?
In the end, most of the Fusion fun kinda fizzled for me, except that is, for any fusion involving a shield.
Shield fusing started out similar to sword fusing for me. I put a shield on my shield, but this time I couldn't help but notice the numbers didn't seem to actually change after the fusions. I saw the calculation claim to add the two together, but the final stat was somehow just the original stat of the base shield instead of the combined sum. In terms of durability, the Stone Talus hearts seemed to withstand a few more hits, but not enough that I could tell for sure. It was only when I discovered on this very site that shields are at their best when fused with random items that it completely changed how I play the game.
Beforehand, I barely cared that I had a shield - you can't even block with one if you're holding a two-handed weapon or bow after all - but then I read about slapping gemstones on them and how that creates a magic trap for the enemies. Don't use the shield to block getting hit, use it to dare the enemy into hurting themselves! Or use a puffshroom, so they instead get lost in a fog created by attacking you. Those Zonai devices that spurt fire and lightning just seem to outright work much better on your shield than they do as a club too.
How about running away from the fight? Or maybe you're just about to climb something real high and want a boost first? Strap a rocket to your shield, and upon blocking Link is sent high into the air, flying off as far as your battery will allow! Even if you cancel it early and lose the rocket, your shield will likely stay intact, so just slap on another one after you land.
Don't have a rocket? The Octoballons aren't perfect, but they will do the same trick, just slower. It completely replaces using a pinecone in a fire, especially since these tricks will still work even when it's raining.
Many of us remember shield sledding from Breath Of The Wild, which felt a bit anticlimactic as you slid down a hill for about half a foot before your shield exploded and Link fell to his death off a cliff. Tears Of The Kingdom quietly lets your shield slide for a longer period of time, and you can make that time even longer by fusing a sled or cart to it.
Plus, like balloons and rockets, the different items are effectively just better or worse versions of each other. Sleds only work if you start uphill, while carts have wheels that push you forward and even give you a bit more control in going left or right. Oh, and mine carts will work the same as Zonai carts, as well as both of them letting you rail-grind like you're Sonic or Ratchet.
But those aren't even the best shield fusions. Do yourself a favor and make sure, whenever you find yourself in an enemy camp, run over to the explosive barrels and fuse as many of them as you can to your shields. Just like the gemstones, tricking an enemy into attacking a bomb-shield will lead to their own destruction. The same area-of-effect from a bomb barrel explosion will happen, so they will likely take out their friends with them, while Link somehow remains unscathed. Normally your shield isn't even that powerful, but somehow this works every time. And yes, a bombflower is an equal substitute if you don't mind dipping into your stash.
Some explosives start a timer after a hit, and fusing won't change that, which is even funnier honestly. You haven't played Tears Of The Kingdom until Link walks up to a bunch of Bokoblins as a time bomb gives them a warning they just ignore. Jump on the bomb, and Link gets sent flying into the air, leaving you the chance to quickly pull out your bow for slow-mo shots as the enemies burn beneath you.
You can keep your Nuts And Bolts mechas. I'll stick with my bomb-shields and cart-shields and rocket-shields. And I think you should too.