Solved it. Looks like the issue was I was using the digital hall sensors instead of the analogue ones. Looking on this https://learn.tinycircuits.com/Wirelings/Hall-Effect_Wireling_Tutorial/ you can see the two sensors do vary slightly (the analogue one has a larger chip). Pacman 30th Anniversary

Just posting my reply here in case this helps anyone else out in the future.

Seeing "Received NACK on transmit of address" is frustrating! Have you tried shortening your I²C wires? I had a similar issue once, and simply using shorter, higher-quality wires solved the problem, seemingly addressing some signal degradation.

Could signal degradation be the issue?

Having trouble finding the right battery too? I ordered some online that claimed to be compatible, but the connector was totally wrong! I ended up going to a local electronics repair shop and they hooked me up with the perfect one. Might be worth a try!