I did that commute for a yr. I chose BWI because of train option to get home when NYC airports close.
MDW has non stops on us. To get to/from BWI you have to go off line. Delta has a 9pm out of JFK that’s always open. I’d take that for my AM flight the next day. For a PM report delta has. 8-9 am flight that gets you in in time. Also United has a flight out of EWR in morning that gets you in in time for PM RAP.
Coming home almost always was delayed. If an AM flight or reserve delta and AA had flights mid day and afternoon. The last flight out was the United at 1930 but sometimes it’s was on a dash8 100 so weight limited but I was only affected by this once.
Like I said when the NYC airports close in the winter there is always the train $80-100 one way last min. If you booked when you got schedule it’s around $40 and refundable so you could go that route. Train is awesome. 2:45 and never crowded. Can get to from train by free shuttle at BWI.
I only came home through ISP a few times. Our flights don’t correspond with train sched so definitely last option.
A few times we had a ferry to LGA. jumped on that. (Best option)

All that being said I moved to a base and wouldn’t look back.

Gents, quick question for the group. I'm looking to live in the MKE area, no SWA base but MDW is close-ish and drivable, although through Chicago which can be obviously rough. Are there MDW trips that originate out of MKE regularly/at all? I know SWA is doing more and more out of MKE lately.

Trips originate at the domiciles.

There are many pilots driving from Wisconsin to MDW. You can even sit reserve at home if you don't live too far north. Traffic can be a PITA in the late afternoon, I-294 and I-55 get busy. MKE is also a good place to non-rev on SWA, as we have many flights out of that airport, however, we do not serve MKE/MDW. You will for the most part be driving to MDW as already mentioned trips originate out domiciles and there will be no trips that start with a MDW-MKE deadhead.

how senior/junior is atl as a base?

You're new here, right. This has been asked and answered many times.

ATL is the most senior base, by an order of magnitude. Years to get there. Many years.

The "plug" FO was hired in September 2010...

So it's getting much more junior then it was. Like others have said, it might tun in a base like Orlando. Top heavy with a few spots on the bottom for junior people.

Is initial seniority done by age, or SS#, or?

Age within your class.