When I have a lot of channelling out, these days (although it's also very much the same as before, anyway), I use ramps instead on the Z-below.
Initial Start Perhaps Safe to Then Until... Tidy up
#_____# #_____# # # # # # # # # # # # #
####### #RRRRR# #^_^_^# #^_^_^# # # # # # # # #
####### ####### ####### #RRRRR# #^___^# # # # # # #
####### ####### ####### ####### #RRRRR# #___^^# # # # #
####### => ####### => ####### => ####### => ####### => #RRRRR# = > #^___^# => #_____#
_ = Flat floor; # = Rock; R = Designate ramp; ^ = Surviving ramp[1]
Do this to as low as you want (or are able!), and to tidy up you just need to (d)esignate (z)"remove ramp or stairs" as and where necessary (almost always against shaft edges, unless you've dug into a pre-existing semi-open area).
Just so you know, I think a true "deep fort" should eschew any proper entrance-way, trap-filled maze or no. I'd adapt my method of well-digging, which is as follows:
Initial Start Digging Ooh! Not yet Stunned
trapped for mo.
_____ __C__ _@C__ _@ __ __ __ __W__ __W__ __W__ To get well __W__ You can then
##### ##C## ##C## ##^## ##@## ## ## ## ## ## ## operational, ##|## even build a
##### ##C## ##C## ##C## ##C## ##@## ## ## ## ## clear any of ##|## wall at the
##### ##C## ##C## ##C## ##C## ##C## ## ## ## ## the debris ##|## lower escape
.etc. .etc. .etc. .etc. .etc. .etc. .. .. .. .. from the ^ ..|.. tunnel for
.etc. .etc. .etc. .etc. .etc. .etc. .. .. .. .. that you want ..|.. neatness.
##### ##C## ##C## ##C## ##C## ##C## ## ## ## ## and then make ##|##
##### => ##### => ##### => ##### => ##### => ##### =etc, etc, until=> ##^TT => ##^__ the final ##|__
channel |
~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~~~~ ~~u~~
_ = Flat floor; # = Rock; C = Designate channel; ^ = Surviving ramp+channel designation!; ~ = Water, with a gap above
@ = Dwarf (obscuring a ramp+designated channel, at all levels but the surface, where obscuring flat floor)
W = Well (can be built any time after dwarf's first two channels, once they're not getting out that way
T = Horizontal digging enabled, off to a stairwell (current or designated!)
using this method I get a nice essentially unbroken well-shaft into a known body of water.
If it's not too far down, and the miner isn't likely to get desperately hungry or thirsty by delaying his downward travel, I sometimes only designate them one channel at a time, enable Engraving for the miner as well and let him smooth the eight bordering tiles at each level before making him dig himself into the next Z-level down. But as my miners are usually entirely rookies with stone smoothing, that often takes time (at least at first) and starvation and dehydration kick in for any well deeper than half a dozen layers or so (especially if the escape-way also needs to be dug out again by same miner!).
Anyway, I'd adapt this plan by setting up a Burrow on a 1x1-tile spot, but spanning multiple vertical levels, and assigning all dwarfs to it. As you miner digs his way down, everyone goes with it. (For bonus marks, Quantum Dump your wagon's goods (and deconstructed materials!) onto the same spot beforehand, so it all travels down with you.)
If you haven't already surveyed the lower levels[2] (or Revealed, or save-scummed back from an initial foray or two!) it takes a leap of faith that you don't find yourself falling into one of your various caverns (or, in your case, just the one) or straight into the magma sea, but if you gamble (or cheat) right you end up with them at the bottom of a drop that nothing that does not fly could possibly deliberately navigate through (although a scuffle between various creatures, e.g. mystified immigrants and hostile wildlife, could end up with something living (initially) dodging its way over your shaft and doing the Wile E Coyote "Whoops"-sign trick for a moment before plummeting an almost-certainly lethal distance down said shaft), and then set yourself up in as completely a self-contained nuclear bombshelterdeep fort as you can carve out for yourself.
[1] Ramps also dissapear (to "_", above) if they're later completely surrounded by ramps, but if a tile is as yet undug and surrounded by ramps then a being ramp-dug it will turn it into a ramp even while it de-ramps the final "dependant" ramps that were adjacent to it (including diagonally along the Z-level). A ramp directly underdug by a new ramp has no effect. However it certainly used to be true that ramping underneath a full-grown tree would cause a cave-in, and you can also do other silly things. Which you'll learn about.
[2] I tend to dig stairwells down, most of the time they hit the cavern roofs and reveal the possible water supplies, and places where further stairwells can be sunk to find the lower caverns by going through the centre of a pillar/cavern wall at least 3x3 wide. Or I might find that I must have already done this, inadvertently, and have to send another stairwell down to see if I can find the missed caverns. The main danger is (at least while I'm still not prepared to let things from the caverns walk straight into my fort) I only find the caverns by setting a stairwell in the side-wall of a bit of cavern with only 1-Z of height, meaning I need to do some quick channelling and/or walling-up to stop my idle/web-seeking dwarfs wandering into the caverns and the nasties wandering out. Later on, I'll have trained protection on stand-by, and maybe one or two other tricks up my sleave.