Just looking at the schedule, if the Cubs don't skip anyone we're looking at these starters the rest of the year:
ARI - Steele, Hendricks, Wicks
PIT - Assad, Taillon, Steele
COL - Hendricks, Wicks, Assad
ATL - Taillon, Steele, Hendricks
MIL - Wicks, Assad, Taillon
But with 3 off days coming up they have a chance skip 1-2 starts to optimize their pitching matchups. Maybe something like (number of days of rest in parenthesis)
ARI - Steele (5), Hendricks (5), Wicks (5)
PIT - Taillon (5), Steele (4), Hendricks (4)
COL - Wicks (4), Taillon (4), Stroman/Assad/BP
ATL - Steele (5), Hendricks (5), Wicks (5)
MIL - Taillon (5), Stroman/Assad/BP (5), Steele (5)
If the Cubs have clinched a playoff spot they can rest Steele on the last game of the season and Hendricks and Wicks well rested for Games 2 and 3. Maybe Stroman back by next Sunday is too soon, not sure but consider that a 5th starter spot for whoever takes it. Obviously if its Taillon you want to pull from the rotation instead of Assad, then that would switch around a bit but same principle. You can basically skip 2 starts from your worst starter down the stretch and still give all of your starters an extra day of rest before all of their final starts except one.