Are there any? I have a few things I'd like to toy with to make DF work the way I want and/or to just get around some problems (as I see it) in the game.
Not that I know of; most of us are largely self-taught. We try things, look around the forum for people who have already done similar things, and ask around to see what does and doesn't work. The wiki is also very helpful; once you know the syntax of various things, it's relatively simple to simply look through the tokens lists and find what it is you need to do what you want.
As Apollo said, looking at existing mods to figure out what needs to go into something helps massively. The vanilla raws are also decent, though they don't really do anything particularly special. Modders have really stretched the limits of what the game is capable of at the moment.
Basically, I want more trade partners, bigger caravans,
Adding trade partners means adding extra civilization entities, which can be very difficult to do successfully. However, there are many preexisting mods that add functional civs, so either using those directly or cannibalizing them for your own purposes can be a good way to get your feet wet.
and some way to make ore type stones last longer.
Ideally I'd like to make ore tiles very slow to mine but drop multiple ore "rocks", if that's even possible... maybe I should try a custom smelter reaction? Make ore "rocks" spit out more bars, would that work?
Doable, but it's a big job. You'd have to add a custom reaction for either each and every ore, or use the ore classes for the reactions. The second is easier, but has undesirable consequences, such as tetrahedrite being 100% certain to produce silver instead of 20%, and if you used it to make silver you wouldn't get copper at all. The first way doesn't have those problems, but you'd need many more reaction entries to cover every ore type.
Also, it's not possible to make ores slower to mine or less likely to drop things. It's one ore per wall dug out max, with fewer produced with lower miner skill. And since miner skill improves so rapidly, well, there's really no reliable way of decreasing stone drops.
I would be OK with more hostile civs, but don't really care to mess about with dozens of new ores and plants.... especially the ores.
All mods are modular to an extent, so if you wish to take the civs out without the plants or ores or non-sapient creatures you can; there can be errors involved, but if you know what you're doing it's not too bad. There are also mods that do just add civs without other things. Relentless Assault, for instance, adds a half-dozen or so new civs, most of which are hostile by default.
Also, is there any way to make invaders drop semi-random items on death, for my DiabloII style amusement?
Not really. ITEMCORPSE makes a creature drop a specific item instead of their actual body on death, but that's the same through the whole species, and invaders don't carry much besides weapons, armor, and clothing. For the most part, the stuff invaders bring isn't even valid for you to use, so that makes true "loot" from enemies difficult to mod in.