Assuming the drive writes normally a simple command like

dd if=/dev/random of=/dev/sdX

Where sdX is the location of the drive should do the trick. Depending on drive time this may take a bit.

This was a few drives ago but there was a point in time when most places were giving me digital copies of tax documents which I could upload to tax prep software but things like TurboTax didn’t have an auto import. So you’d need to download them then re-upload them to the correct service. Now they do it automatically so the only thing that would match that now now is receipts for expenses/donations and what not that I need to keep track of for manual entry.

I started encrypting once I moved to having a decent number of solid state drives as the tech can theoretically leave blocks unerased once they go bad. Before that my primary risk factor was at end of life recycling which I usually did early so I wasn’t overly concerned about tax documents/passwords etc being left as I’d use dd to write over the platters prior to recycling.