I think in modules. it comes from my manufacturing background
one module per stair, think of a stair as 1 shift register. or node.
one NANO per stair, one power supply per stair,
two RJ45 connectors to pass the master signals.
If your power supply can do more than one stair, then you can have a 2-stair module, or a 3-stair module....
of course, you would also need line voltage to each power supply, so that big adds complexity.
now you can do 3 stairs or 33 or 333
your master only has to send - start and end.
or get fancy and have pre-sets and call out
pre-set1 it may have all the lights on dim and slowly light
pre-set2 stair one goes off, then back on after 1/10 second chase up or down staircase
This means the master does the timing for each stair to start and if you use pre-sets, it picks which to use.
IMHO, you can make one stair module and get it to work, then make copies, the only difference is the address.
the master only needs to know how many stair modules.
since the cost of a NANO in the clown market is so cheap, you have to weigh your time vs purchases.