My number used to belong to an elderly woman, so I keep getting spam texts intended for her. I block the numbers, but they somehow keep sending me spam messages from different ones.

I don't think there is any solution other than changing my phone number at this point. The issue is fucking sites keep using phone number as 2fa.

Keep the old number and get a new one. You'll have to gradually switch all 2fa if you want to lose the old number, otherwise keep it forever on a separate device.