A central elevator is very good for mobility, ease of access and future planning. A single tile may do the trick, but leave room for expansion in this central shaft. If you can introduce a waterfall through it later it can become a useful tool to prevent tantrumspirals, as well as a mechanic to clean your populace swiftly to prevent FUN from dangerous blood.
There are many ways to dispose of the bins, if you're looking for a weaponized version then I would suggest building a bridge (non retracting) and then construct a small platform from which you dump things from above onto the bridge. You may need to monitor the weight as to not overburden the bridge, but executed properly you can use this as a junk chucker. Inaccurate, but amusing. Alternitavely do the same thing anywhere else and just atom smash them.
On the caverns I concur with Deboche, you are more than ready to open them up and delve deeper. Underground pastures makes animal management easier, less travel time for your dwarfs. Although, I have yet to create a working tree farm in 40.xx, perhaps allowing for multiple Z levels would help?
As far as siege difficulty, eventually you may incur the wrath of a general who can be a severe threat or a chump. But more insidious threats are the RNG blessed foes who may prove nigh invulnerable. A bronze colossus for instance, has traditionally proven quite effective at holding his own against axe dwarfs.
If you seek magma, you do not have to go to it. Magma pumps, mine arts, and magma pistons are all techniques you can use. Plus, these methods lend themselves more readily to weaponization and utility.