A Simons to Detroit trade isn't going to involve Harris, the point of trading Simons would be for the C's to shed money and get under the tax and Harris does not help with that.
The easiest trade would be Simons to Detroit for Levert and Ivey with one of those two getting flipped elsewhere and Boston keeping the other. That would get Boston under the tax, but it depends on how Detroit feels about Ivey right now.
If Detroit wanted to keep Ivey they could also do Lavert/Reed/Javonte Green and that would work as a salary match. Again, Boston would reroute either Lavert or Reed/Green to a third team and get under the tax.
Stewart could also be involved in these sort of swaps instead of Lavert, he makes a low enough salary that if you swap Simons for Stewart the C's get below the tax. But I really don't think Detroit wants to trade Stewart, and I certainly don't think they should give him up for Simons. If the Pistons are making a win now move I don't think he's available, if he was it would involve the Celtics also sending draft picks.
Pistons could also be a trade destination for Hauser. They have their full MLE and enough space below the tax to just absorb him outright.
Pistons have a lot of flexibility because they still have 19 million in space below the tax, its also worth noting these deals can't happen until at least December 15th because of player trade restrictions. I d think they are an interesting target in a Simons trade, just because we saw how well a flamethrowing shooter worked for them last year with Beasley. Simons would fill that same roll.