What changes after quitting work for good at 53?

[Tkydon]

But reduction in credits also reduces your entitlement.
The Full Quote from the Pension Site, that I condensed says

Benefit Amount (Fiscal year 2021)

￥780,900/ year (full benefit amount based on 40 years (aged between 20 and 59 years) of fully contributed coverage periods)

If you have coverage periods for which you failed to pay or your contribution was exempted, the amount is;
￥780,900x ( (1) + (2) + (3) + (4) + (5) ) / 40 years * × 12
(1) Number of contribution-paid months (no exemption)
(2) Number of full contribution-exempted months** × 1/2
(3) Number of three-quarter contribution-exempted months** × 5/8
(4) Number of half contribution-exempted months** × 3/4
(5) Number of one-quarter contribution-exempted months** × 7/8
* Shorter for some people, depending on date of birth
** Depending on your income or according to the National Pension Act, you may be granted an exemption of full- or partial- amount of contribution payment.

If your contribution is exempted for April 2009 and earlier, please apply the formula below;
(2) Number of full contribution-exempted months × 1/3
(3) Number of three-quarter contribution-exempted months × 1/2
(4) Number of half contribution-exempted months × 2/3
(5) Number of one-quarter contribution-exempted months × 5/6


However, whether they allow you to claim exemptions is up to them, and you would still be liable for the contributions at what ever level they decide.