Yes but when was this decided? I have no doubts that following the 2019 season in January and February of 2020 he wasn't going to sign another contract even if It was until 50. The question is when did Brady come to this conclusion. Because what I am talking about is the during the 2019 off-season and prior.
I think Brady made the decision after Super Bowl LIII in the 2019 offseason based on the available indications:
1) The "contract extension" signed in 2019 was purposefully designed to allow Brady to hit free agency in 2020 with 2020 and 2021 being void years while preventing the Patriots from using the franchise / transition tags.
2) Kraft in the series gave to protocol that he gave Brady a promise that he'll facilitate him being able to leave if he wants in the 2018 offseason, meaning the 2019 "contract extension" was essentially Kraft making good on this promise.
3) Hoyer said recently in a podcast that Brady alluded to him in the 2019 offseason of being prepared to sit out the entire 2019 season shortly before the "extension" got signed.
4) Kraft also gave to protocol in the series that after Super Bowl LIII Brady realized that Belichick would be the head coach for the foreseeable future.
5) In the NFL's official investigation report on Stephen Ross' tampering, it's said that communications between the Dolphins and Brady began as early as August 2019.
Putting conjecture together I think the following happened after the Patriots won Super Bowl LIII:
• Brady's relationship with Belichick was irreparably damaged after 2017 and after they won Super Bowl LIII Brady probably felt (maybe for the first time ever) that winning did not provide any relief to his tensions and issues with Belichick.
• After the performance of the defense in Super Bowl LIII Kraft made it clear to Brady that following that performance he was either genuinely not willing, or too afraid of the public backlash, to move on from Belichick.
• Seeing this, Brady then started to explore his options and reminded Kraft of his promise to help facilitate Brady's exit from New England if he desires so, which he acquiesced to through that "extension".
• Then there's also Brady's attitude throughout the 2019 season, especially when he publicly described himself to be the most miserable 8-0 quarterback ever after the Patriots blew out the Browns. That was quite out of character for him as he was in this case publicly and explicitly putting himself above the team. To me back then it was quite a weird statement to make and not helpful at all in the quest to win another championship. Looking back, I think it's a sign that by then he mentally had already moved on which affected the degree of his commitment to the team.