Summary
In the Orlando bubble, Mills acted more like an assistant coach than a player, which made it seem like the Spurs were handing the keys over to their young guards like Dejounte Murray, Derrick White, and Lonnie Walker. However, the Spurs waited another year to do that as Mills showed in '20-'21 that he can still contribute as a player, and continued that level of play with the Nets in '21-'22.

Mills is a microwave scorer off the bench, hitting 41% from three for his career and 40% in '21-'22, and letting it fly whenever he gets an opening. Mills was very good as a scoring threat, especially when Brooklyn was short-handed. He did cool off as the season progressed, which has been a consistent theme over the past few years, but the fact that he still finished as a 40% three-point shooter highlights on scorching hot he was early in the season.

Despite being 33 years old, Mills still moves well and has played nearly 30 minutes per game in '21-'22. Due to his size, Mills doesn't thrive on defense, but he's not a complete target that some players of his stature are. Mills has never been an elite playmaker or someone that you want running pick-and-rolls, but he fits in perfectly with second units or teams with wings that operate as primary ball-handlers. If Mills opts out, many contenders will likely have interest in him again in 2022.
 
Cap Considerations
Mills has a $6.2 million player option for next season, and if he declines that option he will be an unrestricted free agent with a $7.1 million cap hold and Non-Bird Rights. If Mills opts for free agency, the Nets will be somewhat limited in their means of re-signing him. Since they will only have his Non-Bird Rights, the most they could offer him using the Non-Bird Exception is a contract starting at $7.1 million. This should be enough to re-sign Mills, but since the Nets will have limited resources to replace Mills if he leaves, he may be able to use that as leverage to get the full amount allowed under the Non-Bird Exception.

Other teams will have multiple ways to sign Mills as even teams without cap space could possibly use the Non-Taxpayer Mid-Level Exception or Taxpayer MLE to sign him if he opts out, but it's hard to imagine he would opt out and settle for the Room MLE or less.