I realized I solved this problem, but never finished up this thread. Parts of the solution might help someone else, so here goes. It's a bit of a hack. But a clean hack, as hacks go.
To recap: what I needed was the glue board(s) between the mill, the steppers, their drivers, and my Mesa card. I'm not ready to switch to something like the TB6600 for the drivers, and even if I did, I'd still need mounting hardware, a new box, etc. My shop is very small -- my mill basically fills the space it's in, up against a wall, tucked in between the workbench and some shelving. No room for big new boxes. Only small new boxes.
I searched for carrier boards which would hold multiple Polulu style driver modules, but came up empty. Lots of carriers for just one, but none for four of them. Seems odd, but I looked pretty hard.
What I ended up doing is leveraging the MKS router controller that came with the new mill. It had the power distribution I needed, the sockets for the A4988 modules, headers that matched the stepper motor wiring, of course all of which I needed. It also had the Atmel processor running Grbl, laser outputs, and some other stuff, which I didn't need.
And very conveniently, and unexpected, there is a header next to each Polulu socket which gives access to the step/dir/enable signals between the processor and each driver module. I think these are probably there so that you could use the controller to drive external stepper drivers instead of the Polulus, but I realized that if I simply kept the processor in reset, it would be tri-stated, and I could instead use those headers as external inputs to the A4988 drivers. In other words, I could use my existing controller as the "carrier" board I'd been looking for, with no modifications at all. (The board has a header which makes the board reset signal available, normally connected e-stop. I just jumpered it to ground.)
So that's what I did. I have a (very small) box sitting next to the mill that contains the original mill controller (relocated -- it used to be mounted on the gantry), and the Mesa card. The pre-made cables from the motors to the controller card were all long enough to reach the box, with no splicing. (Yes, I know cardboard isn't optimum. But getting it all working and enclosed came first. I'll make a better box soon.)
I still needed a small interface board to make wiring the limit switches more tidy. They're all too short to reach the Mesa directly. At Adafruit, I found a small breakout board, intended for an RPi pico. It's basically a bunch of screw terminals, .1" headers, and solderable grid, in a compact form factor, all for just $12.50:
www.adafruit.com/product/5907
. That went into a small project box mounted on the gantry, and serves as the junction box where the limit switches, e-stop, and probe inputs all gather, and are wired in a bundle to the Mesa. It gave me an organized place to join the +/- limit switches into single inputs, etc, but mostly it just a place to screw down wires in order to connect them together.
I know it wouldn't be everyone's design, but I'm happy with it for now, and it went together cleanly.
