(For digging a pit, I've found that the recent addition of job priorities helps the micromanagement if I let dwarves (suitably sated for food/drink/sleep prior to being sent off to the job) self-trap themselves in a pit as they dig, but I tend to have a temporary wallside ramp access to the top (and then to the bottom, and that exit, when completed) whilst slowly arranging a new level cut out (sans ramp) and its sides fully smoothed prior to arranging the next level, but you seem to have that bit sorted, so you don't need to take heed of my fuss.)
I usually make exits to pits be a central (single ramp-adjacented) undug rock pillar up a level that a drawbridge lowers over to from the internal access. No reason, at least prior to climbing abilities mattering, but it just always seemed useful.
Whether or not you do that, at the next level above your bottom layer or such protuberances as just described, arange an all-covering retracting bridge as the nominal landing zone might help. You lose one or more levels of fall (or need to dig one or more levels beneath the target one), but pulling a lever can deposit dead and dying and injured (and merely annoyed?) victims into the area below, then extend the bridge over again for more uncushioned falling-fun whilst the work safely goes on below to clean up the prior results. (I think making the bridge of dense material like platinum will do you more good than lthe less dense ones like featherwood.)
Add in judicious arrow-slit fortifications in the pit walls above and below such a barrier (to deal with the awkwardly robust, at one point or another in the process) and maybe some other little tricks of the trade (optional pumping of your choice of liquid into and out of various bits1 of the pit-tube?) and a combinatorial killing (or not!) pit with many optional methods to its operation can be usefully engineered...
1 Remember to make sure arrow-slits are shutterable, if you do this where they might interact...