I see a lot of reviews on Amazon about aftermarket batteries that claim crazy mAh ratings and die after a few months.

This one looks promising from some reviews in Germany, but I haven't actually bought it yet. Reviews here are scant and shitty.

I'd love to find a reputable aftermarket battery.

https://www.amazon.com/Lachy-Upgraded-Capacity-Replacement-Motorhead/dp/B09ZL8Z3PQ