So, the update:
PG: Reggie Jackson / Ish Smith / Lorenzo Brown
SG: Kentavious Caldwell-Pope / Reggie Bullock / Michael Gbinije
SF: Marcus Morris / Stanley Johnson / Darrun Hilliard
PF: Tobias Harris / Jon Leuer / Henry Ellenson
C: Andre Drummond / Aron Baynes / Boban Marjanovic
The roster now stands at 15 players under (or soon to be) contract. By my sheet, which is linked below, the Pistons have roughly just over $2.5 million in cap space. That's assuming Mayo's earlier report that Blake has been renounced as well as Anthony and Bairstow being released is true. They're also around $8 million under the luxury tax with Drummond's extension included so they're pretty safe in that regard. The cap hits in the first year for Smith and Leuer are just estimates at this point, and nobody knows how much of his draft slot Ellenson has or will sign for yet. I think it's safe to say Gbinije is only getting a minimum contract but that's still up in the air as well.
With all that in mind the only "easy" move the Pistons have left to open up cap space is cutting Lorenzo Brown and his unguaranteed contract. If they do that there will be roughly $3.6 million in cap space available. Of course Brown may very well end up keeping his roster spot as the third point guard since that's pretty much the only move left Detroit would make. Not a ton of space to absorb someone decent in a trade even with Brown cut, and it seems pretty unlikely Bullock or Hilliard would go out in one unless a rotation-quality combo guard is coming back. Still plenty of point guards left on the market though and it seems like things are drying up a bit for money being handed out.
I should also mention that the Pistons still have the room exception to use as well. Off the top of my head it's something like $2.898 million for the first season and then whatever it goes up to for the season(s) after if it's for more than one year. I'm pretty sure Zaza Pachulia, Seth Curry, and DeWayne Dedmon were signed using it so decent (or at least intriguing) players have come off the board via that route this summer.
https://docs.google.com/spreadsheets/d/1DWFeG156MPjSSd3aOjewz5WzQbc6AiWOqgrOHc2TebM/edit?usp=sharing