Summary
Hartenstein made a somewhat bold bet in the summer of 2021, opting out of his guaranteed minimum contract with Cleveland. It looked to be a mistake at first, as Hartenstein had to settle for a non-guaranteed minimum contract with the Clippers. However, he made the roster and had a very nice season with the Clippers, establishing himself as a quality backup center, and it led to him signing a 2-year, $16 million contract with the Knicks in 2022, where he has significantly outplayed that contract.

Hartenstein has really nice touch, as he has the ability to hit floaters as the roll man. He can also find shooters if the defense collapses. Hartenstein is also athletic enough to be a lob threat. He has good offensive rebounding numbers, having an above-average offensive rebounding rate each season of his career, according to Cleaning the Glass. He has only hit 66% of his shots at the rim with the Knicks, but overall has been a solid finisher inside throughout his career and his effectiveness from floater range makes him efficient overall. He hasn't yet been able to become a threat from outside, but has a decent stroke and it's not impossible that he could develop that part of his game. 

Defensively, Hartenstein isn't an elite rim protector but posts very good rim protection numbers. He also has been a disruptive defender, consistently posting high steal and block rates throughout his career. He fouls too much, but that's not a fatal flaw as a back-up big that isn't expected to play heavy minutes. It could matter for teams considering him as a starting center option though.

Hartenstein will be 26 years old as a free agent in 2024, so while he's no longer a young prospect that you would expect to make major strides, I wouldn't expect him to regress over the life of his next contract either. 
 
Cap Considerations
Hartenstein will be an unrestricted free agent with an $10.7 million cap hold and Early Bird rights. Since the Knicks will be operating above the salary cap, his cap hold is not really relevant, as it will either be replaced by his starting salary if he re-signs with the Knicks, or removed upon signing with another team. The Knicks could offer Hartenstein a starting salary of up to $16.8 million using the Early Bird Exception, which should be enough to re-sign him, but I could also see another team making him a larger offer.

For other teams, if Hartenstein demands a starting salary of more than the full $12.9 million Non-Taxpayer Mid-Level Exception, only teams with cap space will be able to sign him, unless a sign-and-trade is worked out. It's possible that Hartenstein could be had for the full MLE or slightly less, but I would consider that unlikely.