Hard to tell with the graphics pack (might be significantly different from default graphics, if what I think is being asked is being asked) but by "mine out a floor without becoming accessible" you mean channelling it away?
If so, then you've started it wrong, I think. You've mined (designate to dig) horizontally on this layer and (at least) the layer below, and are now removing by channelling (designate as channel) the floor left between. If you'd instead been able to ignore this level (or dig it out, if you want, but not necessary) then designate ramps on the untouched level below then they could have been serviced (as soon as accessible, but from any accessible spot without prejudice) it would open up 'this' level if still needed whilst removing this unwanted floor.
Then when that sublayer of ramps is completely dug, designate the untouched layer below for ramps (if desired) and that level gets opened up and removes the prior ramp-layer (and its flooring) in the process, just as safely.
(There are ways to mess this up, but only by getting impatient and overconfident and trying to do more than one new layer each time. Or punching into any voids. But this looks close enough to the surface to not to hit anything like that.)
As it is, how I think you're currently situated, prioritised digging/micromanagement is the way to go. If you give priority 1,3,5,7 to one (retreating) floor-removal and 2,4,6 to the opposing one, two miners would 'claim' two different orderly retreats back to the safe access point.
Or just designate each safe channelling as it becomes safe (miners may commute away and then back, slowing things down).
Or use a civ burrow over another area (or levelk(s) needing clearing) to send all but one active miner off to dig elsewhere and keep this single miner working on this level's retreat by a civ burrow here solely for them.
Or variations/combinations. I know what I'd do in (what I think is) your situation, but explaining it in a single post isn't always easy, as you can see.
If I'm wrong about the channel-the-floor-away thing, this may not be any help at all. Or it may help in the future.