Thanks, gents!
Seems stable now, and it *might* be ready to move on to cutting air - here's an update. Any thoughts?
One of the issues I was having earlier was that I'd test moves in MDI and not get consistent behaviors. I was *assuming* that moves would be incremental. Nope. I think there may be a random-mode-default generator in the background.

Once I started setting G91 explicitly in MDI, that settled down and I started to get the incrementals that I expected.
The mill currently passes the simple test of moving a given distance on an axis, and then returning the same distance, to its hard stop. This for all of XYZA.
I did take advice to just bump the axes at the hard stops and pass on using switches. That's the advice in the wiki for hobby mills as well - just took me a while to find it. Yes, the steppers make some noise if they stall, but apparently, no real harm done.
This is a Taig 2019CR-ER mill, so the table's a bit larger than the original.
X Axis
The motor and its stop are on the right side of the table, so from the stop position (table hard left), the only way the table and workpiece can move, is to the right.
Home = 0.0
MIN_LIMIT = -11.75
MAX_LIMIT = 0.0
HOME_OFFSET = 0.0
Y Axis
The motor and its stop are on the front of the table, so from the stop position (table hard to the front), the only way the table and workpiece can move, is inward, toward the Z pillar.
Home = 0.0
MIN_LIMIT = 0.0
MAX_LIMIT = 5.0
HOME_OFFSET = 0.0
Z Axis
The motor and its stop are at the top of the Z pillar, so from the stop position (headstock hard up), the only way the headstock and tool can move, is downward, toward the table.
Home = 0.0
MIN_LIMIT = -0.0
MAX_LIMIT = 6.0
HOME_OFFSET = 0.0
One oddity, is that for the Z axis to pass a move-out-then-move-back-home test for some given distance, the MIN_LIMIT had to be "-0.0", because "0.0" would fail with a complaint about hitting its soft limit. That got solved, but it's peculiar...
Thx!
- Bob