Flights from either ATL or JFK to Milan how much time to allow between our originating flights

If it is a single ticket, Delta has figured out how much time is needed.

If you are buying 2 tickets, less than 3hrs may be more drama than necessary. If you have checked luggage add an hour.

Delta has only allowed 1 hr

If that is what Delta allows then it should work since they are responsible for your travel.

Currently the DL flight from JFK to MXP departs at 20:45
I think you will have only one choice, the Delta flight which leaves SEA at 7:15 and arrives at JFK at 15:24 because the flight after that won’t get you to New York on time to catch the flight to Milan.
Currently there are no flights from ATL to MXP, but there is probably one in summer. Even in that case the flight will likely leave in the evening, and from SEA to JFK Delta probably has no more than 4 or 5 flights, only one of which is likely to get you to KFK on time, and from the first coast it means the first flight out of SEA in the morning.

I routed from SEA to Milan via CDG in October and it worked out fine. I had 3 hours or so in CDG which was plenty of time. (And actually, of course it was Spokane->Seattle->CDG->Milan). But that would mean if you wanted to do your long leg with your friend they'd need to fly to Seattle.

Trying to decide between the one hour layovers vs. the 3+ hr ones Delta offers?

If you want to minimize your time sitting in an airport, you could book the 1 hr connection and have the later flight as a fall back if your 1st flight is late. ( assuming there are empty seats )

If you want to reduce the stress factor, book the 3hr connection choice. Personally arriving a little later in the morning in Milan and not have the stress of a 1 hr connection would be my choice. (presumably this is the Sept/Oct trip mentioned in another of your posts?)

Thank you we’re booked thru JFK with comfortable amount of time between flights