Advice on national pension retroactive payment

Apologies if this has been asked before.

I've been living in Japan since 2017. From July 2019 to July 2022, I have been exempt from paying Kokumin-nenkin premiums due to being enrolled in a PhD program. Now, the pension system authorities have offered me to contribute retroactively to make up for these "lost years" (国民健康保険料追納).

Does it make sense from a financial standpoint to do so? The way I see it, the national pension system seems more like a ponzi scheme due to demographics and the way the system is constructed. Am I right in my understanding that the monthly Kokumin-nenkin benefits are a basic flat-rate and will not improve regardless how much I have contributed, as long as I am over the 10 year threshold?

I tend to think making these payments will not significantly improve my pension benefits down the road and I'd be wise to take the same amount and invest it into something more effective.

For what its worth, I'm a German citizen, which has a bilateral social security agreement with Japan.

Thanks!