Curious... Why do you need to do agriculture on the surface?
I like Rope Reeds (he's an elf!), using Whip Vine Flour for meals (ELF!), and the additional dye plants (EEEEEELF!!!!!) available at the surface to make valuable meal stacks and cloth goods to trade (EEEEEELFFFFFFF!!!!!!). The stunning variety and value of booze available from surface plants incl. the only WHISKEY (perhaps he IS a dwarf?) in the game makes me stick to the surface.
On top of this, there's the "elephants don't inhabit caverns" and "tame crocodiles behind door #1 sound fun" sorts of reasons to generally have a surface presence
of some sort. (Yes, you
can get a breeding pair of those animals from the elves, Virginia, but you might hit the lottery, too.)
Further, I don't understand why there's such a worry about letting traders make a walk to the deep. Deep doesn't mean "let them inside fort security", it just refers to what relative Z-level you're talking about. The difference between 100 and 50 steps to the depot isn't really that much difference, really, and you can always force traders to enter by an alternate route from your dwarves using pressure plates and hatches. Put a drawbridge "at the back" of the Depot, and let dwarves in from the back while you defend the entry tunnel with traps (not cages, something lethal without reloading). With that plan, you could keep invaders out (raised bridge over dry moat) and let traders in, while retaining your fort at magma-forge level and forcing traders to walk all the way down there. Sure, they may trigger (or lead) an ambush deep into the tunnels, but a 50-Z set of ramps on switchbacks, with each level full of traps you don't have to spend much time or effort to maintain is pretty dwarfy and doesn't have any FPS death issues that can come from pumping magma in some cases.
A "bridge over dry moat" built into one of your switchbacks allows you to seal the area off from the outside to prevent anything from coming in while you're trading.
For extra security, build a few areas full of lever-triggered spikes and put them all on one lever. You can take the lever off repeat when the traders pass through the area, and put it on when there's no one coming - and everyone who's at the point of "build your whole fort in the deeps" probably can afford to spare dwarves for lever duty. That'll pretty much hash
anything that gets into the area.
Yes, there's a high setup cost to a trap hallway that long, and it requires working around the caverns and holding off their inhabitants while you get it built, but large architecture projects that can't be 100% pre-planned are dwarfy too, right?