Is TechTool Pro worth the money?

Perhaps nine or ten years ago, Apple used to include TechTool Pro with Mac purchases. Presumably there was some sort of arrangement between the two companies, and the TechTool Pro tool was a useful diagnostic tool. Then that arrangement ended. Personally I kept purchasing TechTool Pro upgrades for a year or two later. But then it seemed the TechTool Pro tool failed to keep up with something - perhaps it was some hardware, firmware, or software changes on Apple's part. I had issues when having TechTool Pro installed. Removing the product solved the issues and I have not used them since.

Apple does provide their own hardware diagnostics if you need them. Such things as safe mode and Apple Diagnostics are good enough for me.

Try safe mode if your Mac doesn't finish starting up - Apple Support

Using Apple Diagnostics - Apple Support

I would save your money.

Are you having issues that need resolved?

I too am a frequent "defragger" in days past. There was a Windows tool called "Diskeeper" that I used regularly. But Apple has always tended to disk upkeep, at least since I started using Apple machines about ten years ago. They used to defrag in the middle of the night - I'm not sure when or how it occurs now.