I'm very rusty here, so please correct me where I'm wrong.

Isn't most of the radiation that makes it to the earth's surface from the sun just EM radiation? That acts a lot different than radiation due to nuclear decay. Your use of the unit 'tons' makes me think you're talking about particle radiation, of which the only one that reaches earth's surface in large quantities would be muons, which may as well be ignored because they aren't interacting with anything.

The water being released by Japan has the following isotopes:

All four of these isotopes decay via beta decay.

So, a comparison to the Sun seems weird here.

Here's an IAEA overview as of February 2023,

The discharge of the ALPS treated water into the sea will be conducted after i) purification/re-purification to meet regulatory standards set based on international standards with an exception of tritium and ii) to allay the concerns of the consumers, the target concentration of tritium should be the same as the operational target (less than 1,500 Bq/L, that is less than 1/40 of the regulatory standard value for tritium) by sufficient dilution (more than 100 times) by sea water, prior to the discharge into the sea, and iii) The total annual amount of tritium to be discharged will be at a level below the operational target value for tritium discharge of the Fukushima Daiichi NPS before the accident (22 trillion Bq/year).

So it's diluted well below internationally accepted concentrations. Moreover, the release is even less than when it was operational!